Letters of a Radio-Engineer to His Son
by John Mills
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[Transcriber's Note: An underscore character "" is used around text to signify italics in the original text, as illustrated. It also is used to signify a subscript, used frequently in technical descriptions. For example E{C} would have been originally typeset as a capital E followed by a smaller C subscript, and both would have been in an italic typeface.]




Engineering Department, Western Electric Company, Inc.,

Author of "Radio-Communication," "The Realities of Modern Science," and "Within the Atom"









J. M., Jr.


1 Electricity and Matter 3

2 Why a Copper Wire Will Conduct Electricity 9

3 How a Battery Works 16

4 The Batteries in Your Radio Set 27

5 Getting Electrons from a Heated Wire 34

6 The Audion 40

7 How to Measure an Electron Stream 48

8 Electron-Moving-Forces 57

9 The Audion-Characteristic 66

10 Condensers and Coils 77

11 A "C-W" Transmitter 86

12 Inductance and Capacity 96

13 Tuning 112

14 Why and How to Use a Detector 124

15 Radio-Telephony 140

16 The Human Voice 152

17 Grid Batteries and Grid Condensers for Detectors 165

18 Amplifiers and the Regenerative Circuit 176

19 The Audion Amplifier and Its Connections 187

20 Telephone Receivers and Other Electromagnetic Devices 199

21 Your Receiving Set and How to Experiment 211

22 High-Powered Radio-Telephone Transmitters 230

23 Amplification at Intermediate Frequencies 242

24 By Wire and by Radio 251

Index 263


I One of the Lines of Towers at Radio Central Frontispiece

II Bird's-Eye View of Radio Central 10

III Dry Battery for Use in Audion Circuits, and also Storage Battery 27

IV Radiotron 42

V Variometer and Variable Condenser of the General Radio Company. Voltmeter and Ammeter of the Weston Instrument Company 91

VI Low-Power Transmitting Tube, U V 202 106

VII Photographs of Vibrating Strings 155

VIII To Illustrate the Mechanism for the Production of the Human Voice 170

IX Western Electric Loud Speaking Receiver. Crystal Detector Set of the General Electric Co. Audibility Meter of General Radio Co. 203

X Audio-Frequency Transformer and Banked-Wound Coil 218

XI Broadcasting Equipment, Developed by the American Telephone and Telegraph Company and the Western Electric Company 235

XII Broadcasting Station of the American Telephone and Telegraph Company on the Roof of the Walker-Lispenard Bldg. in New York City where the Long-distance Telephone Lines Terminate 250





You are interested in radio-telephony and want me to explain it to you. I'll do so in the shortest and easiest way which I can devise. The explanation will be the simplest which I can give and still make it possible for you to build and operate your own set and to understand the operation of the large commercial sets to which you will listen.

I'll write you a series of letters which will contain only what is important in the radio of to-day and those ideas which seem necessary if you are to follow the rapid advances which radio is making. Some of the letters you will find to require a second reading and study. In the case of a few you might postpone a second reading until you have finished those which interest you most. I'll mark the letters to omit in this way.

All the letters will be written just as I would talk to you, for I shall draw little sketches as I go along. One of them will tell you how to experiment for yourself. This will be the most interesting of all. You can find plenty of books to tell you how radio sets operate and what to do, but very few except some for advanced students tell you how to experiment for yourself. Not to waste time in your own experiments, however, you will need to be quite familiar with the ideas of the other letters.

What is a radio set? Copper wires, tinfoil, glass plates, sheets of mica, metal, and wood. Where does it get its ability to work—that is, where does the "energy" come from which runs the set? From batteries or from dynamos. That much you know already, but what is the real reason that we can use copper wires, metal plates, audions, crystals, and batteries to send messages and to receive them?

The reason is that all these things are made of little specks, too tiny ever to see, which we might call specks of electricity. There are only two kinds of specks and we had better give them their right names at once to save time. One kind of speck is called "electron" and the other kind "proton." How do they differ? They probably differ in size but we don't yet know so very much about their sizes. They differ in laziness a great deal. One is about 1845 times as lazy as the other. That is, it has eighteen hundred and forty-five times as much inertia as the other. It is harder to get it started but it is just as much harder to get it to stop after it is once started or to change its direction and go a different direction. The proton has the larger inertia. It is the electron which is the easier to start or stop.

How else do they differ? They differ in their actions. Protons don't like to associate with other protons but take quite keenly to electrons. And electrons—they go with protons but they won't associate with each other. An electron always likes to be close to a proton. Two is company when one is an electron and the other a proton but three is a crowd always.

It doesn't make any difference to a proton what electron it is keeping company with provided only it is an electron and not another proton. All electrons are alike as far as we can tell and so are all protons. That means that all the stuff, or matter, of our world is made up of two kinds of building blocks, and all the blocks of each kind are just alike. Of course you mustn't think of these blocks as like bricks, for we don't know their shapes.

Then there is another reason why you must not think of them as bricks and that is because when you build a house out of bricks each brick must rest on another. Between an electron and any other electron or between two protons or between an electron and a proton there is usually a relatively enormous distance. There is enough space so that lots of other electrons or protons could be fitted in between if only they were willing to get that close together.

Sometimes they do get very close together. I can tell you how if you will imagine four small boys playing tag. Suppose Tom and Dick don't like to play with each other and run away from each other if they can. Now suppose that Bill and Sam won't play with each other if they can help it but that either of them will play with Tom or Dick whenever there is a chance. Now suppose Tom and Bill see each other; they start running toward each other to get up some sort of a game. But Sam sees Tom at the same time, so he starts running to join him even though Bill is going to be there too. Meanwhile Dick sees Bill and Sam running along and since they are his natural playmates he follows them. In a minute they are all together, and playing a great game; although some of the boys don't like to play together.

Whenever there is a group of protons and electrons playing together we have what we call an "atom." There are about ninety different games which electrons and protons can play, that is ninety different kinds of atoms. These games differ in the number of electrons and protons who play and in the way they arrange themselves. Larger games can be formed if a number of atoms join together. Then there is a "molecule." Of molecules there are as many kinds as there are different substances in the world. It takes a lot of molecules together to form something big enough to see, for even the largest molecule, that of starch, is much too small to be seen by itself with the best possible microscope.

What sort of a molecule is formed will depend upon how many and what kinds of atoms group together to play the larger game. Whenever there is a big game it doesn't mean that the little atomic groups which enter into it are all changed around. They keep together like a troop of boy scouts in a grand picnic in which lots of troops are present. At any rate they keep together enough so that we can still call them a group, that is an atom, even though they do adapt their game somewhat so as to fit in with other groups—that is with other atoms.

What will the kind of atom depend upon? It will depend upon how many electrons and protons are grouped together in it to play their little game. How any atom behaves so far as associating with other groups or atoms will depend upon what sort of a game its own electrons and protons are playing.

Now the simplest kind of a game that can be played, and the one with the smallest number of electrons and protons, is that played by a single proton and a single electron. I don't know just how it is played but I should guess that they sort of chase each other around in circles. At any rate I do know that the atom called "hydrogen" is formed by just one proton and one electron. Suppose they were magnified until they were as large as the moon and the earth. Then they would be just about as far apart but the smaller one would be the proton.

That hydrogen atom is responsible for lots of interesting things for it is a great one to join with other atoms. We don't often find it by itself although we can make it change its partners and go from one molecule to another very easily. That is what happens every time you stain anything with acid. A hydrogen atom leaves a molecule of the acid and then it isn't acid any more. What remains isn't a happy group either for it has lost some of its playfellows. The hydrogen goes and joins with the stuff which gets stained. But it doesn't join with the whole molecule; it picks out part of it to associate with and that leaves the other part to take the place of the hydrogen in the original molecule of acid from which it came. Many of the actions which we call chemistry are merely the result of such changes of atoms from one molecule to another.

Not only does the hydrogen atom like to associate in a larger game with other kinds of atoms but it likes to do so with one of its own kind. When it does we have a molecule of hydrogen gas, the same gas as is used in balloons.

We haven't seemed to get very far yet toward radio but you can see how we shall when I tell you that next time I shall write of more complicated games such as are played in the atoms of copper which form the wires of radio sets and of how these wires can do what we call "carrying an electric current."




You have learned that the simplest group which can be formed by protons and electrons is one proton and one electron chasing each other around in a fast game. This group is called an atom of hydrogen. A molecule of hydrogen is two of these groups together.

All the other possible kinds of groups are more complicated. The next simplest is that of the atom of helium. Helium is a gas of which small quantities are obtained from certain oil wells and there isn't very much of it to be obtained. It is an inert gas, as we call it, because it won't burn or combine with anything else. It doesn't care to enter into the larger games of molecular groups. It is satisfied to be as it is, so that it isn't much use in chemistry because you can't make anything else out of it. That's the reason why it is so highly recommended for filling balloons or airships, because it cannot burn or explode. It is not as light as hydrogen but it serves quite well for making balloons buoyant in air.

This helium atom is made up of four electrons and four protons. Right at the center there is a small closely crowded group which contains all the protons and two of the electrons. The other two electrons play around quite a little way from this inner group. It will make our explanations easier if we learn to call this inner group "the nucleus" of the atom. It is the center of the atom and the other two electrons play around about it just as the earth and Mars and the other planets play or revolve about the sun as a center. That is why we shall call these two electrons "planetary electrons."

There are about ninety different kinds of atoms and they all have names. Some of them are more familiar than hydrogen and helium. For example, there is the iron atom, the copper atom, the sulphur atom and so on. Some of these atoms you ought to know and so, before telling you more of how atoms are formed by protons and electrons, I am going to write down the names of some of the atoms which we have in the earth and rocks of our world, in the water of the oceans, and in the air above.

Start first with air. It is a mixture of several kinds of gases. Each gas is a different kind of atom. There is just a slight trace of hydrogen and a very small amount of helium and of some other gases which I won't bother you with learning. Most of the air, however, is nitrogen, about 78 percent in fact and almost all the rest is oxygen. About 20.8 percent is oxygen so that all the gases other than these two make up only about 1.2 percent of the atmosphere in which we live.

The earth and rocks also contain a great deal of oxygen; about 47.3 percent of the atoms which form earth and rocks are oxygen atoms. About half of the rest of the atoms are of a kind called silicon. Sand is made up of atoms of silicon and oxygen and you know how much sand there is. About 27.7 percent of the earth and its rocks is silicon. The next most important kind of atom in the earth is aluminum and after that iron and then calcium. Here is the way they run in percentages: Aluminum 7.8 percent; iron 4.5 percent; calcium 3.5 percent; sodium 2.4 percent; potassium 2.4 percent; magnesium 2.2 percent. Besides these which are most important there is about 0.2 percent of hydrogen and the same amount of carbon. Then there is a little phosphorus, a little sulphur, a little fluorine, and small amounts of all of the rest of the different kinds of atoms.

Sea water is mostly oxygen and hydrogen, about 85.8 percent of oxygen and 10.7 percent of hydrogen. That is what you would expect for water is made up of molecules which in turn are formed by two atoms of hydrogen and one atom of oxygen. The oxygen atom is about sixteen times as heavy as the hydrogen atom. However, for every oxygen atom there are two hydrogen atoms so that for every pound of hydrogen in water there are about eight pounds of oxygen. That is why there is about eight times as high a percentage of oxygen in sea water as there is of hydrogen.

Most of sea water, therefore, is just water, that is, pure water. But it contains some other substances as well and the best known of these is salt. Salt is a substance the molecules of which contain atoms of sodium and of chlorine. That is why sea water is about 1.1 percent sodium and about 2.1 percent chlorine. There are some other kinds of atoms in sea water, as you would expect, for it gets all the substances which the waters of the earth dissolve and carry down to it but they are unimportant in amounts.

Now we know something about the names of the important kinds of atoms and can take up again the question of how they are formed by protons and electrons. No matter what kind of atom we are dealing with we always have a nucleus or center and some electrons playing around that nucleus like tiny planets. The only differences between one kind of atom and any other kind are differences in the nucleus and differences in the number and arrangement of the planetary electrons which are playing about the nucleus.

No matter what kind of atom we are considering there is always in it just as many electrons as protons. For example, the iron atom is formed by a nucleus and twenty-six electrons playing around it. The copper atom has twenty-nine electrons as tiny planets to its nucleus. What does that mean about its nucleus? That there are twenty-nine more protons in the nucleus than there are electrons. Silver has even more planetary electrons, for it has 47. Radium has 88 and the heaviest atom of all, that of uranium, has 92.

We might use numbers for the different kinds of atoms instead of names if we wanted to do so. We could describe any kind of atom by telling how many planetary electrons there were in it. For example, hydrogen would be number 1, helium number 2, lithium of which you perhaps never heard, would be number 3, and so on. Oxygen is 8, sodium is 11, chlorine is 17, iron 26, and copper 29. For each kind of atom there is a number. Let's call that number its atomic number.

Now let's see what the atomic number tells us. Take copper, for example, which is number 29. In each atom of copper there are 29 electrons playing around the nucleus. The nucleus itself is a little inner group of electrons and protons, but there are more protons than electrons in it; twenty-nine more in fact. In an atom there is always an extra proton in the nucleus for each planetary electron. That makes the total number of protons and electrons the same.

About the nucleus of a copper atom there are playing 29 electrons just as if the nucleus was a teacher responsible for 29 children who were out in the play yard. There is one very funny thing about it all, however, and that is that we must think of the scholars as if they were all just alike so that the teacher couldn't tell one from the other. Electrons are all alike, you remember. All the teacher or nucleus cares for is that there shall be just the right number playing around her. You could bring a boy in from some other play ground and the teacher couldn't tell that he was a stranger but she would know that something was the matter for there would be one too many in her group. She is responsible for just 29 scholars, and the nucleus of the copper atom is responsible for just 29 electrons. It doesn't make any difference where these electrons come from provided there are always just 29 playing around the nucleus. If there are more or less than 29 something peculiar will happen.

We shall see later what might happen, but first let's think of an enormous lot of atoms such as there would be in a copper wire. A small copper wire will have in it billions of copper atoms, each with its planetary electrons playing their invisible game about their own nucleus. There is quite a little distance in any atom between the nucleus and any of the electrons for which it is responsible. There is usually a greater distance still between one atomic group and any other.

On the whole the electrons hold pretty close to their own circles about their own nuclei. There is always some tendency to run away and play in some other group. With 29 electrons it's no wonder if sometimes one goes wandering off and finally gets into the game about some other nucleus. Of course, an electron from some other atom may come wandering along and take the place just left vacant, so that nucleus is satisfied.

We don't know all we might about how the electrons wander around from atom to atom inside a copper wire but we do know that there are always a lot of them moving about in the spaces between the atoms. Some of them are going one way and some another.

It's these wandering electrons which are affected when a battery is connected to a copper wire. Every single electron which is away from its home group, and wandering around, is sent scampering along toward the end of the wire which is connected to the positive plate or terminal of the battery and away from the negative plate. That's what the battery does to them for being away from home; it drives them along the wire. There's a regular stream or procession of them from the negative end of the wire toward the positive. When we have a stream of electrons like this we say we have a current of electricity.

We'll need to learn more later about a current of electricity but one of the first things we ought to know is how a battery is made and why it affects these wandering electrons in the copper wire. That's what I shall tell you in my next letter.[1]

[Footnote 1: The reader who wishes the shortest path to the construction and operation of a radio set should omit the next two letters.]



(This letter may be omitted on the first reading.)


When I was a boy we used to make our own batteries for our experiments. That was before storage batteries became as widely used as they are to-day when everybody has one in the starting system of his automobile. That was also before the day of the small dry battery such as we use in pocket flash lights. The batteries which we made were like those which they used on telegraph systems, and were sometimes called "gravity" batteries. Of course, we tried several kinds and I believe I got quite a little acid around the house at one time or another. I'll tell you about only one kind but I shall use the words "electron," "proton," "nucleus," "atom," and "molecule," about some of which nothing was known when I was a boy.

We used a straight-sided glass jar which would hold about a gallon. On the bottom we set a star shaped arrangement made of sheets of copper with a long wire soldered to it so as to reach up out of the jar. Then we poured in a solution of copper sulphate until the jar was about half full. This solution was made by dissolving in water crystals of "blue vitriol" which we bought at the drug store.

Blue vitriol, or copper sulphate as the chemists would call it, is a substance which forms glassy blue crystals. Its molecules are formed of copper atoms, sulphur atoms, and oxygen atoms. In each molecule of it there is one atom of copper, one of sulphur and four of oxygen.

When it dissolves in water the molecules of the blue vitriol go wandering out into the spaces between the water molecules. But that isn't all that happens or the most important thing for one who is interested in making a battery.

Each molecule is formed by six atoms, that is by six little groups of electrons playing about six little nuclei. About each nucleus there is going on a game but some of the electrons are playing in the game about their own nucleus and at the same time taking some part in the game which is going on around one of the other nuclei. That's why the groups or atoms stay together as a molecule. When the molecules wander out into the spaces between the water molecules something happens to this complicated game.

It will be easiest to see what sort of thing happens if we talk about a molecule of ordinary table salt, for that has only two atoms in it. One atom is sodium and one is chlorine. The sodium molecule has eleven electrons playing around its nucleus. Fairly close to the nucleus there are two electrons. Then farther away there are eight more and these are having a perfect game. Then still farther away from the nucleus there is a single lonely electron.

The atom of chlorine has seventeen electrons which play about its nucleus. Close to the nucleus there are two. A little farther away there are eight just as there are in the sodium atom. Then still farther away there are seven.

I am going to draw a picture (Fig. 1) to show what I mean, but you must remember that these electrons are not all in the same plane as if they lay on a sheet of paper, but are scattered all around just as they would be if they were specks on a ball.

You see that the sodium atom has one lonely electron which hasn't any play fellows and that the chlorine atom has seven in its outside circle. It appears that eight would make a much better game. Suppose that extra electron in the sodium atom goes over and plays with those in the chlorine atom so as to make eight in the outside group as I have shown Fig. 2. That will be all right as long as it doesn't get out of sight of its own nucleus because you remember that the sodium nucleus is responsible for eleven electrons. The lonely electron of the sodium atom needn't be lonely any more if it can persuade its nucleus to stay so close to the chlorine atom that it can play in the outer circle of the chlorine atom.

The outer circle of the chlorine atom will then have a better game, for it will have just the eight that makes a perfect game. This can happen if the chlorine atom will stay close enough to the sodium atom so that the outermost electron of the sodium atom can play in the chlorine circle. You see everything will be satisfactory if an electron can be shared by the two atoms. That can happen only if the two atoms stay together; that is, if they form a molecule. That's why there are molecules and that's what I meant when I spoke of the molecule as a big game played by the electrons of two or more atoms.

This molecule which is formed by a sodium atom and a chlorine atom is called a molecule of sodium chloride by chemists and a molecule of salt by most every one who eats it. Something strange happens when it dissolves. It wanders around between the water molecules and for some reason or other—we don't know exactly why—it decides to split up again into sodium and chlorine but it can't quite do it. The electron which joined the game about the chlorine nucleus won't leave it. The result is that the nucleus of the sodium atom gets away but it leaves this one electron behind.

What gets away isn't a sodium atom for it has one too few electrons; and what remains behind isn't a chlorine atom for it has one too many electrons. We call these new groups "ions" from a Greek word which means "to go" for they do go, wandering off into the spaces between the water molecules. Fig. 3 gives you an idea of what happens.

You remember that in an atom there are always just as many protons as electrons. In this sodium ion which is formed when the nucleus of the sodium atom breaks away but leaves behind one planetary electron, there is then one more proton than there are electrons. Because it has an extra proton, which hasn't any electron to associate with, we call it a plus ion or a "positive ion." Similarly we call the chlorine ion, which has one less proton than it has electrons, a minus or "negative ion."

Now, despite the fact that these ions broke away from each other they aren't really satisfied. Any time that the sodium ion can find an electron to take the place of the one it lost it will welcome it. That is, the sodium ion will want to go toward places where there are extra electrons. In the same way the chlorine ion will go toward places where electrons are wanted as if it could satisfy its guilty conscience by giving up the electron which it stole from the sodium atom, or at least by giving away some other electron, for they are all alike anyway.

Sometimes a positive sodium ion and a negative chlorine ion meet in their wanderings in the solution and both get satisfied by forming a molecule again. Even so they don't stay together long before they split apart and start wandering again. That's what goes on over and over again, millions of times, when you dissolve a little salt in a glass of water.

Now we can see what happens when copper sulphate dissolves. The copper atom has twenty-nine electrons about its nucleus and all except two of these are nicely grouped for playing their games about the nucleus. Two of the electrons are rather out of the game, and are unsatisfied. They play with the electrons of the part of the molecule which is called "sulphate," that is, the part formed by the sulphur atom and the four oxygen atoms. These five atoms of the sulphate part stay together very well and so we treat them as a group.

The sulphate group and the copper atom stay together as long as they are not in solution but when they are, they act very much like the sodium and chlorine which I just described. The molecule splits up into two ions, one positive and one negative. The positive ion is the copper part except that two of the electrons which really belong to a copper atom got left behind because the sulphate part wouldn't give them up. The rest of the molecule is the negative ion.

The copper ion is a copper atom which has lost two electrons. The sulphate ion is a combination of one sulphur atom, four oxygen atoms and two electrons which it stole from the copper atom. Just as the sodium ion is unsatisfied because in it there is one more proton than there are electrons, so the copper ion is unsatisfied. As a matter of fact it is twice as badly unsatisfied. It has two more protons than it has electrons. We say it has twice the "electrical charge" of the sodium ion.

Just like a sodium ion the copper ion will tend to go toward any place where there are extra electrons which it can get to satisfy its own needs. In much the same way the sulphate ion will go toward places where it can give up its two extra electrons. Sometimes, of course, as ions of these two kinds wander about between the water molecules, they meet and satisfy each other by forming a molecule of copper sulphate. But if they do they will split apart later on; that is, they will "dissociate" as we should say.

Now let's go on with the kind of batteries I used to make as a boy. You can see that in the solution of copper sulphate at the bottom of the jar there was always present a lot of positive copper ions and of negative sulphate ions.

On top of this solution of copper sulphate I poured very carefully a weak solution of sulphuric acid. As I told you, an acid always has hydrogen in its molecules. Sulphuric acid has molecules formed by two hydrogen atoms and one of the groups which we decided to call sulphate. A better name for this acid would be hydrogen sulphate for that would imply that its molecule is the same as one of copper sulphate, except that the place of the copper is taken by two atoms of hydrogen. It takes two atoms of hydrogen because the copper atom has two lonely electrons while a hydrogen atom only has one. It takes two electrons to fill up the game which the electrons of the sulphate group are playing. If it can get these from a single atom, all right; but if it has to get one from each of two atoms, it will do it that way.

I remember when I mixed the sulphuric acid with water that I learned to pour the acid into the water and not the other way around. Spatterings of sulphuric acid are not good for hands or clothes. With this solution I filled the jar almost to the top and then hung over the edge a sort of a crow's foot shape of cast zinc. The zinc reached down into the sulphuric acid solution. There was a binding post on it to which a wire could be connected. This wire and the one which came from the plate of copper at the bottom were the two terminals of the battery. We called the wire from the copper "positive" and the one from the zinc "negative."

Now we shall see why and how the battery worked. The molecules of sulphuric acid dissociate in solution just as do those of copper sulphate. When sulphuric acid molecules split, the sulphate part goes away with two electrons which don't belong to it and each of the hydrogen atoms goes away by itself but without its electron. We call each a "hydrogen ion" but you can see that each is a single proton.

In the two solutions are pieces of zinc and copper. Zinc is like all the rest of the metals in one way. Atoms of metals always have lonely electrons for which there doesn't seem to be room in the game which is going on around their nuclei. Copper as we saw has two lonely electrons in each atom. Zinc also has two. Some metals have one and some two and some even more lonely electrons in each atom.

What happens then is this. The sulphate ions wandering around in the weak solution of sulphuric acid come along beside the zinc plate and beckon to its atoms. The sulphate ions had a great deal rather play the game called "zinc sulphate" than the game called "hydrogen sulphate." So the zinc atoms leave their places to join with the sulphate ions. But wait a minute! The sulphate ions have two extra electrons which they kept from the hydrogen atoms. They don't need the two lonely electrons which each zinc atom could bring and so the zinc atom leaves behind it these unnecessary electrons.

Every time a zinc atom leaves the plate it fails to take all its electrons with it. What leaves the zinc plate, therefore, to go into solution is really not a zinc atom but is a zinc ion; that is, it is the nucleus of a zinc atom and all except two of the planetary electrons.

Every time a zinc ion leaves the plate there are left behind two electrons. The plate doesn't want them for all the rest of its atoms have just the same number of protons as of electrons. Where are they to go? We shall see in a minute.

Sometimes the zinc ions which have got into solution meet with sulphate ions and form zinc sulphate molecules. But if they do these molecules split up sooner or later into ions again. In the upper part of the liquid in the jar, therefore, there are sulphate ions which are negative and two kinds of positive ions, namely, the hydrogen ions and the zinc ions.

Before the zinc ions began to crowd in there were just enough hydrogen ions to go with the sulphate ions. As it is, the entrance of the zinc ions has increased the number of positive ions and now there are too many. Some of the positive ions, therefore, and particularly the hydrogen ions, because the sulphate prefers to associate with the zinc ions, can't find enough playfellows and so go down in the jar.

Down in the bottom of the jar the hydrogen ions find more sulphate ions to play with, but that leaves the copper ions which used to play with these sulphate ions without any playmates. So the copper ions go still further down and join with the copper atoms of the copper plate. They haven't much right to do so, for you remember that they haven't their proper number of electrons. Each copper ion lacks two electrons of being a copper atom. Nevertheless they join the copper plate. The result is a plate of copper which has too few electrons. It needs two electrons for every copper ion which joins it.

How about the zinc plate? You remember that it has two electrons more than it needs for every zinc ion which has left it. If only the extra electrons on the negative zinc plate could get around to the positive copper plate. They can if we connect a wire from one plate to the other. Then the electrons from the zinc stream into the spaces between the atoms of the wire and push ahead of them the electrons which are wandering around in these spaces. At the other end an equal number of electrons leave the wire to satisfy the positive copper plate. So we have a stream of electrons in the wire, that is, a current of electricity and our battery is working.

That's the sort of a battery I used to play with. If you understand it you can get the general idea of all batteries. Let me express it in general terms.

At the negative plate of a battery ions go into solution and electrons are left behind. At the other end of the battery positive ions are crowded out of solution and join the plate where they cause a scarcity of electrons; that is, make the plate positive. If a wire is connected between the two plates, electrons will stream through it from the negative plate to the positive; and this stream is a current of electricity.



(This letter may be omitted on the first reading.)


You will need several batteries when you come to set up your radio receiver but you won't use such clumsy affairs as the gravity cell which I described in my last letter. Some of your batteries will be dry batteries of the size used in pocket flash lights.

These are not really dry, for between the plates they are filled with a moist paste which is then sealed in with wax to keep it from drying out or from spilling. Instead of zinc and copper these batteries use zinc and carbon. No glass jar is needed, for the zinc is formed into a jar shape. In this is placed the paste and in the center of the paste a rod or bar of carbon. The paste doesn't contain sulphuric acid, but instead has in it a stuff called sal ammoniac; that is, ammonium chloride.

The battery, however, acts very much like the one I described in my last letter. Ions of zinc leave the zinc and wander into the moist paste. These ions are positive, just as in the case of the gravity battery. The result is that the electrons which used to associate with a zinc ion to form a zinc atom are left in the zinc plate. That makes the zinc negative for it has more electrons than protons. The zinc ions take the place of the positive ions which are already in the paste. The positive ions which originally belonged with the paste, therefore, move along to the carbon rod and there get some electrons. Taking electrons away from the carbon leaves it with too many protons; that is, leaves it positive. In the little flash light batteries, therefore, you will always find that the round carbon rod, which sticks out of the center, is positive and the zinc casing is negative.

The trouble with the battery like the one I used to make is that the zinc plate wastes away. Every time a zinc ion leaves it that means that the greater part of an atom is gone. Then when the two electrons which were left behind get a chance to start along a copper wire toward the positive plate of the battery there goes the rest of the atom. After a while there is no more zinc plate. It is easy to see what has happened. All the zinc has gone into solution or been "eaten away" as most people say. Dry batteries, however, don't stop working because the zinc gets used up, but because the active stuff in the paste, the ammonium chloride, is changed into something else.

There's another kind of battery which you will need to use with your radio set; that is the storage battery. Storage batteries can be used over and over again if they are charged between times and will last for a long time if properly cared for. Then too, they can give a large current, that is, a big swift-moving stream of electrons. You will need that when you wish to heat the filament of the audion in your receiving set.

The English call our storage batteries by the name "accumulators." I don't like that name at all, but I don't like our name for them nearly as well as I do the name "reversible batteries." Nobody uses this last name because it's too late to change. Nevertheless a storage battery is reversible, for it will work either way at an instant's notice.

A storage battery is something like a boy's wagon on a hill side. It will run down hill but it can be pushed up again for another descent. You can use it to send a stream of electrons through a wire from its negative plate to its positive plate. Then if you connect these plates to some other battery or to a generator, (that is, a dynamo) you can make a stream of electrons go in the other direction. When you have done so long enough the battery is charged again and ready to discharge.

I am not going to tell you very much about the storage battery but you ought to know a little about it if you are to own and run one with your radio set. When it is all charged and ready to work, the negative plate is a lot of soft spongy lead held in place by a frame of harder lead. The positive plate is a lead frame with small squares which are filled with lead peroxide, as it is called. This is a substance with molecules formed of one lead atom and two oxygen atoms. Why the chemists call it lead peroxide instead of just lead oxide I'll tell you some other time, but not in these letters.

Between the two plates is a wood separator to keep pieces of lead from falling down between and touching both plates. You know what would happen if a piece of metal touched both plates. There would be a short circuit, that is, a sort of a short cut across lots by which some of the electrons from the negative plate could get to the positive plate without going along the wires which we want them to travel. That's why there are separators.

The two plates are in a jar of sulphuric acid solution. The sulphuric acid has molecules which split up in solution, as you remember, into hydrogen ions and the ions which we called "sulphate." In my gravity battery the sulphate ions used to coax the zinc ions away into the solution. In the storage battery on the other hand the sulphate ions can get to most of the lead atoms because the lead is so spongy. When they do, they form lead sulphate right where the lead atoms are. They don't really need whole lead atoms, because they have two more electrons than they deserve, so there are two extra electrons for every molecule of lead sulphate which is formed. That's why the spongy lead plate is negative.

The lead sulphate won't dissolve, so it stays there on the plate as a whitish coating. Now see what that means. What are the hydrogen ions going to do? As long as there was sulphuric acid in the water there was plenty of sulphate ions for them to associate with as often as they met; and they would meet pretty often. But if the sulphate ions get tied up with the lead of the plate there will be too many hydrogen ions left in the solution. Now what are the hydrogen ions to do? They are going to get as far away from each other as they can, for they are nothing but protons; and protons don't like to associate. They only stayed around in the first place because there was always plenty of sulphate ions with whom they liked to play.

When the hydrogen ions try to get away from each other they go to the other plate of the battery, and there they will get some electrons, if they have to steal in their turn.

I won't try to tell you all that happens at the other plate. The hydrogen ions get the electrons which they need, but they get something more. They get some of the oxygen away from the plate and so form molecules of water. You remember that water molecules are made of two atoms of hydrogen and one of oxygen. Meanwhile, the lead atoms, which have lost their oxygen companions, combine with some of the sulphate ions which are in that neighborhood. During the mix-up electrons are carried away from the plate and that leaves it positive.

The result of all this is a little lead sulphate on each plate, a negative plate where the spongy lead was, and a positive plate where the lead peroxide was.

Notice very carefully that I said "a little lead sulphate on each plate." The sort of thing I have been describing doesn't go on very long. If it did the battery would run down inside itself and then when we came to start our automobile we would have to get out and crank.

How long does it go on? Answer another question first. So far we haven't connected any wire between the two plates of the battery, and so none of the electrons on the negative plate have any way of getting around to the positive plate where electrons are badly needed. Every time a negative sulphate ion combines with the spongy lead of the negative plate there are two more electrons added to that plate. You know how well electrons like each other. Do they let the sulphate ions keep giving that plate more electrons? There is the other question; and the answer is that they do not. Every electron that is added to that plate makes it just so much harder for another sulphate ion to get near enough to do business at all. That's why after a few extra electrons have accumulated on the spongy lead plate the actions which I was describing come to a stop.

Do they ever begin again? They do just as soon as there is any reduction in the number of electrons which are hopping around in the negative plate trying to keep out of each other's way. When we connect a wire between the plates we let some of these extra electrons of the negative plate pass along to the positive plate where they will be welcome. And the moment a couple of them start off on that errand along comes another sulphate ion in the solution and lands two more electrons on the plate. That's how the battery keeps on discharging.

We mustn't let it get too much discharged for the lead sulphate is not soluble, as I just told you, and it will coat up that plate until there isn't much chance of getting the process to reverse. That's why we are so careful not to let the discharge process go on too long before we reverse it and charge. That's why, when the car battery has been used pretty hard to start the car, I like to run quite a while to let the generator charge the battery again. When the battery charges, the process reverses and we get spongy lead on the negative plate and lead peroxide on the positive plate.

You've learned enough for one day. Write me your questions and I'll answer and then go on in my next letter to tell how the audion works. You know about conduction of electricity in wires; that is, about the electron stream, and about batteries which can cause the stream. Now you are ready for the most wonderful little device known to science: the audion.




I was pleased to get your letter and its questions. Yes, a proton is a speck of electricity of the kind we call positive and an electron is of the kind we call negative. You might remember this simple law; "Like kinds of electricity repel, and unlike attract."

The word ion[2] is used to describe any atom, or part of a molecule which can travel by itself and has more or less than its proper number of electrons. By proper number of electrons I mean proper for the number of protons which it has. If an ion has more electrons than protons it is negative; if the inequality is the other way around it is positive. An atom or molecule has neither more nor less protons than electrons. It is neutral or "uncharged," as we say.

No, not every substance which will dissolve will dissociate or split up into positive and negative ions. The salt which you eat will, but the sugar will not. If you want a name for those substances which will dissociate in solution, call them "electrolytes." To make a battery we must always use an electrolyte.

Yes, it is hard to think of a smooth piece of metal or a wire as full of holes. Even in the densest solids like lead the atoms are quite far apart and there are large spaces between the nuclei and the planetary electrons of each atom.

I hope this clears up the questions in your mind for I want to get along to the vacuum tube. By a vacuum we mean a space which has very few atoms or molecules in it, just as few as we can possibly get, with the best methods of pumping and exhausting. For the present let's suppose that we can get all the gas molecules, that is, all the air, out of a little glass bulb.

The audion is a glass bulb like an electric light bulb which has in it a thread, or filament, of metal. The ends of this filament extend out through the glass so that we may connect a battery to them and pass a current of electricity through the wire. If we do so the wire gets hot.

What do we mean when we say "the wire gets hot?" We mean that it feels hot. It heats the glass bulb and we can feel it. But what do we mean in words of electrons and atoms? To answer this we must start back a little way.

In every bit of matter in our world the atoms and molecules are in very rapid motion. In gases they can move anywhere; and do. That's why odors travel so fast. In liquids most of the molecules or atoms have to do their moving without getting out of the dish or above the surface. Not all of them stay in, however, for some are always getting away from the liquid and going out into the air above. That is why a dish of water will dry up so quickly. The faster the molecules are going the better chance they have of jumping clear away from the water like fish jumping in the lake at sundown. Heating the liquid makes its molecules move faster and so more of them are able to jump clear of the rest of the liquid. That's why when we come in wet we hang our clothes where they will get warm. The water in them evaporates more quickly when it is heated because all we mean by "heating" is speeding up the molecules.

In a solid body the molecules can't get very far away from where they start but they keep moving back and forth and around and around. The hotter the body is, the faster are its molecules moving. Generally they move a little farther when the body is hot than when it is cold. That means they must have a little more room and that is why a body is larger when hot than when cold. It expands with heating because its molecules are moving more rapidly and slightly farther.

When a wire is heated its molecules and atoms are hurried up and they dash back and forth faster than before. Now you know that a wire, like the filament of a lamp, gets hot when the "electricity is turned on," that is, when there is a stream of electrons passing through it. Why does it get hot? Because when the electrons stream through it they bump and jostle their way along like rude boys on a crowded sidewalk. The atoms have to step a bit more lively to keep out of the way. These more rapid motions of the atoms we recognize by the wire growing hotter.

That is why an electric current heats a wire through which it is flowing. Now what happens to the electrons, the rude boys who are dodging their way along the sidewalk? Some of them are going so fast and so carelessly that they will have to dodge out into the gutter and off the sidewalk entirely. The more boys that are rushing along and the faster they are going the more of them will be turned aside and plunge off the sidewalks.

The greater and faster the stream of electrons, that is the more current which is flowing through the wire, the more electrons will be "emitted," that is, thrown out of the wire. If you could watch them you would see them shooting out of the wire, here, there, and all along its length, and going in every direction. The number which shoot out each second isn't very large until they have stirred things up so that the wire is just about red hot.

What becomes of them? Sometimes they don't get very far away from the wire and so come back inside again. They scoot off the sidewalk and on again just as boys do in dodging their way along. Some of them start away as if they were going for good.

If the wire is in a vacuum tube, as it is in the case of the audion, they can't get very far away. Of course there is lots of room; but they are going so fast that they need more room just as older boys who run fast need a larger play ground than do the little tots. By and by there gets to be so many of them outside that they have to dodge each other and some of them are always dodging back into the wire while new electrons are shooting out from it.

When there are just as many electrons dodging back into the wire each second as are being emitted from it the vacuum in the tube has all the electrons it can hold. We might say it is "saturated" with electrons, which means, in slang, "full up." If any more electrons are to get out of the filament just as many others which are already outside have to go back inside. Or else they have got to be taken away somewhere else.

What I have just told you about electrons getting away from a heated wire is very much like what happens when a liquid is heated. The molecules of the liquid get away from the surface. If we cover a dish of liquid which is being heated the liquid molecules can't get far away and very soon the space between the surface of the liquid and the cover gets saturated with them. Then every time another molecule escapes from the surface of the liquid there must be some molecule which goes back into the liquid. There is then just as much condensation back into liquid as there is evaporation from it. That's why in cooking they put covers over the vessels when they don't want the liquid all to "boil away."

Sometimes we speak of the vacuum tube in the same words we would use in describing evaporation of a liquid. The molecules of the liquid which have escaped form what is called a "vapor" of the liquid. As you know there is usually considerable water vapor in the air. We say then that electrons are "boiled out" of the filament and that there is a "vapor of electrons" in the tube.

That is enough for this letter. Next time I shall tell you how use is made of these electrons which have been boiled out and are free in the space around the filament.

[Footnote 2: If the reader has omitted Letters 3 and 4 he should omit this paragraph and the next.]




In my last letter I told how electrons are boiled out of a heated filament. The hotter the filament the more electrons are emitted each second. If the temperature is kept steady, or constant as we say, then there are emitted each second just the same number of electrons. When the filament is enclosed in a vessel or glass bulb these electrons which get free from it cannot go very far away. Some of them, therefore, have to come back to the filament and the number which returns each second is just equal to the number which is leaving. You realize that this is what is happening inside an ordinary electric light bulb when its filament is being heated.

An ordinary electric light bulb, however, is not an audion although it is like one in the emission of electrons from its filament. That reminds me that last night as I was waiting for a train I picked up one of the Radio Supplements which so many newspapers are now running. There was a column of enquiries. One letter told how its writer had tried to use an ordinary electric light bulb to receive radio signals.

He had plenty of electrons in it but no way to control them and make their motions useful. In an audion besides the filament there are two other things. One is a little sheet or plate of metal with a connecting wire leading out through the glass walls and the other is a little wire screen shaped like a gridiron and so called a "grid." It also has a connecting wire leading through the glass. Fig. 4 shows an audion. It will be most convenient, however, to represent an audion as in Fig. 5. There you see the filament, F, with its two terminals brought out from the tube, the plate, P, and between these the grid, G.

These three parts of the tube are sometimes called "elements." Usually, however, they are called "electrodes" and that is why the audion is spoken of as the "three-electrode vacuum tube." An electrode is what we call any piece of metal or wire which is so placed as to let us get at electrons (or ions) to control their motions. Let us see how it does so.

To start with, we shall forget the grid and think of a tube with only a filament and a plate in it—a two-electrode tube. We shall represent it as in Fig. 6 and show the battery which heats the filament by some lines as at A. In this way of representing a battery each cell is represented by a short heavy line and a longer lighter line. The heavy line stands for the negative plate and the longer line for the positive plate. We shall call the battery which heats the filament the "filament battery" or sometimes the "A-battery." As you see, it is formed by several battery cells connected in series.

Sometime later I may tell you how to connect battery cells together and why. For the present all you need to remember is that two batteries are in series if the positive plate of one is connected to the negative plate of the other. If the batteries are alike they will pull an electron just twice as hard as either could alone.

To heat the filament of an audion, such as you will probably use in your set, will require three storage-battery cells, like the one I described in my fourth letter, all connected in series. We generally use storage batteries of about the same size as those in the automobile. If you will look at the automobile battery you will see that it is made of three cells connected in series. That battery would do very well for the filament circuit.

By the way, do you know what a "circuit" is? The word comes from the same Latin word as our word "circus." The Romans were very fond of chariot racing at their circuses and built race tracks around which the chariots could go. A circuit, therefore, is a path or track around which something can race; and an electrical circuit is a path around which electrons can race. The filament, the A-battery and the connecting wires of Fig. 6 form a circuit.

Let us imagine another battery formed by several cells in series which we shall connect to the tube as in Fig. 7. All the positive and negative terminals of these batteries are connected in pairs, the positive of one cell to the negative of the next, except for one positive and one negative. The remaining positive terminal is the positive terminal of the battery which we are making by this series connection. We then connect this positive terminal to the plate and the negative terminal to the filament as shown in the figure. This new battery we shall call the "plate battery" or the "B-battery."

Now what's going to happen? The B-battery will want to take in electrons at its positive terminal and to send them out at its negative terminal. The positive is connected to the plate in the vacuum tube of the figure and so draws some of the electrons of the plate away from it. Where do these electrons come from? They used to belong to the atoms of the plate but they were out playing in the space between the atoms, so that they came right along when the battery called them. That leaves the plate with less than its proper number of electrons; that is, leaves it positive. So the plate immediately draws to itself some of the electrons which are dodging about in the vacuum around it.

Do you remember what was happening in the tube? The filament was steadily going on emitting electrons although there were already in the tube so many electrons that just as many crowded back into the filament each second as the filament sent out. The filament was neither gaining nor losing electrons, although it was busy sending them out and welcoming them home again.

When the B-battery gets to work all this is changed. The B-battery attracts electrons to the plate and so reduces the crowd in the tube. Then there are not as many electrons crowding back into the filament as there were before and so the filament loses more than it gets back.

Suppose that, before the B-battery was connected to the plate, each tiny length of the filament was emitting 1000 electrons each second but was getting 1000 back each second. There was no net change. Now, suppose that the B-battery takes away 100 of these each second. Then only 900 get back to the filament and there is a net loss from the filament of 100. Each second this tiny length of filament sends into the vacuum 100 electrons which are taken out at the plate. From each little bit of filament there is a stream of electrons to the plate. Millions of electrons, therefore, stream across from filament to plate. That is, there is a current of electricity between filament and plate and this current continues to flow as long as the A-battery and the B-battery do their work.

The negative terminal of the B-battery is connected to the filament. Every time this battery pulls an electron from the plate its negative terminal shoves one out to the filament. You know from my third and fourth letters that electrons are carried through a battery from its positive to its negative terminal. You see, then, that there is the same stream of electrons through the B-battery as there is through the vacuum between filament and plate. This same stream passes also through the wires which connect the battery to the tube. The path followed by the stream of electrons includes the wires, the vacuum and the battery in series. We call this path the "plate circuit."

We can connect a telephone receiver, or a current-measuring instrument, or any thing we wish which will pass a stream of electrons, so as to let this same stream of electrons pass through it also. All we have to do is to connect the instrument in series with the other parts of the plate circuit. I'll show you how in a minute, but just now I want you to understand that we have a stream of electrons, for I want to tell you how it may be controlled.

Suppose we use another battery and connect it between the grid and the filament so as to make the grid positive. That would mean connecting the positive terminal of the battery to the grid and the negative to the filament as shown by the C-battery of Fig. 8. This figure also shows a current-measuring instrument in the plate circuit.

What effect is this C-battery, or grid-battery, going to have on the current in the plate circuit? Making the grid positive makes it want electrons. It will therefore act just as we saw that the plate did and pull electrons across the vacuum towards itself.

What happens then is something like this: Electrons are freed at the filament; the plate and the grid both call them and they start off in a rush. Some of them are stopped by the wires of the grid but most of them go on by to the plate. The grid is mostly open space, you know, and the electrons move as fast as lightning. They are going too fast in the general direction of the grid to stop and look for its few and small wires.

When the grid is positive the grid helps the plate to call electrons away from the filament. Making the grid positive, therefore, increases the stream of electrons between filament and plate; that is, increases the current in the plate circuit.

We could get the same effect so far as concerns an increased plate current by using more batteries in series in the plate circuit so as to pull harder. But the grid is so close to the filament that a single battery cell in the grid circuit can call electrons so strongly that it would take several extra battery cells in the plate circuit to produce the same effect.

If we reverse the grid battery, as in Fig. 9, so as to make the grid negative, then, instead of attracting electrons the grid repels them. Nowhere near as many electrons will stream across to the plate when the grid says, "No, go back." The grid is in a strategic position and what it says has a great effect.

When there is no battery connected to the grid it has no possibility of influencing the electron stream which the plate is attracting to itself. We say, then, that the grid is uncharged or is at "zero potential," meaning that it is zero or nothing in possibility. But when the grid is charged, no matter how little, it makes a change in the plate current. When the grid says "Come on," even though very softly, it has as much effect on the electrons as if the plate shouted at them, and a lot of extra electrons rush for the plate. But when the grid whispers "Go back," many electrons which would otherwise have gone streaking off to the plate crowd back toward the filament. That's how the audion works. There is an electron stream and a wonderfully sensitive way of controlling the stream.



(This letter may be omitted on the first reading.)


If we are to talk about the audion and how its grid controls the current in the plate circuit we must know something of how to measure currents. An electric current is a stream of electrons. We measure it by finding the rate at which electrons are traveling along through the circuit.

What do we mean by the word "rate?" You know what it means when a speedometer says twenty miles an hour. If the car should keep going just as it was doing at the instant you looked at the speedometer it would go twenty miles in the next hour. Its rate is twenty miles an hour even though it runs into a smash the next minute and never goes anywhere again except to the junk heap.

It's the same when we talk of electric currents. We say there is a current of such and such a number of electrons a second going by each point in the circuit. We don't mean that the current isn't going to change, for it may get larger or smaller, but we do mean that if the stream of electrons keeps going just as it is there will be such and such a number of electrons pass by in the next second.

In most of the electrical circuits with which you will deal you will find that electrons must be passing along in the circuit at a most amazing rate if there is to be any appreciable effect. When you turn on the 40-watt light at your desk you start them going through the filament of the lamp at the rate of about two and a half billion billion each second. You have stood on the sidewalk in the city and watched the people stream past you. Just suppose you could stand beside that narrow little sidewalk which the filament offers to the electrons and count them as they go by. We don't try to count them although we do to-day know about how many go by in a second if the current is steady.

If some one asks you how old you are you don't say "About five hundred million seconds"; you tell him in years. When some one asks how large a current is flowing in a wire we don't tell him six billion billion electrons each second; we tell him "one ampere." Just as we use years as the units in which to count up time so we use amperes as the units in which to count up streams of electrons. When a wire is carrying a current of one ampere the electrons are streaming through it at the rate of about 6,000,000,000,000,000,000 a second.

Don't try to remember this number but do remember that an ampere is a unit in which we measure currents just as a year is a unit in which we measure time. An ampere is a unit in which we measure streams of electrons just as "miles per hour" is a unit in which we measure the speed of trains or automobiles.

If you wanted to find the weight of something you would take a scale and weigh it, wouldn't you? You might take that spring balance which hangs out in the kitchen. But if the spring balance said the thing weighed five pounds how would you know if it was right? Of course you might take what ever it was down town and weigh it on some other scales but how would you know those scales gave correct weight?

The only way to find out would be to try the scales with weights which you were sure were right and see if the readings on the scale correspond to the known weights. Then you could trust it to tell you the weight of something else. That's the way scales are tested. In fact that's the way that the makers know how to mark them in the first place. They put on known weights and marked the lines and figures which you see. What they did was called "calibrating" the scale. You could make a scale for yourself if you wished, but if it was to be reliable you would have to find the places for the markings by applying known weights, that is, by calibration.

How would you know that the weights you used to calibrate your scale were really what you thought them to be? You would have to find some place where they had a weight that everybody would agree was correct and then compare your weight with that. You might, for example, send your pound weight to the Bureau of Standards in Washington and for a small payment have the Bureau compare it with the pound which it keeps as a standard.

That is easy where one is interested in a pound. But it is a little different when one is interested in an ampere. You can't make an ampere out of a piece of platinum as you can a standard pound weight. An ampere is a stream of electrons at about the rate of six billion billion a second. No one could ever count anywhere near that many, and yet everybody who is concerned with electricity wants to be able to measure currents in amperes. How is it done?

First there is made an instrument which will have something in it to move when electrons are flowing through the instrument. We want a meter for the flow of electrons. In the basement we have a meter for the flow of gas and another for the flow of water. Each of these has some part which will move when the water or the gas passes through. But they are both arranged with little gear wheels so as to keep track of all the water or gas which has flowed through; they won't tell the rate at which the gas or water is flowing. They are like the odometer on the car which gives the "trip mileage" or the "total mileage." We want a meter like the speedometer which will indicate at each instant just how fast the electrons are streaming through it.

There are several kinds of meters but I shall not try to tell you now of more than one. The simplest to understand is called a "hot-wire meter." You already know that an electron stream heats a wire. Suppose a piece of fine wire is fastened at the two ends and that there are binding posts also fastened to these ends of the wire so that the wire may be made part of the circuit where we want to know the electron stream. Then the same stream of electrons will flow through the fine wire as through the other parts of the circuit. Because the wire is fine it acts like a very narrow sidewalk for the stream of electrons and they have to bump and jostle pretty hard to get through. That's why the wire gets heated.

You know that a heated wire expands. This wire expands. It grows longer and because it is held firmly at the ends it must bow out at the center. The bigger the rate of flow of electrons the hotter it gets; and the hotter it gets the more it bows out. At the center we might fasten one end—the short end—of a little lever. A small motion of this short end of the lever will mean a large motion of the other end, just like a "teeter board" when one end is longer than the other; the child on the long end travels further than the child on the short end. The lever magnifies the motion of the center of the hot wire part of our meter so that we can see it easier.

There are several ways to make such a meter. The one shown in Fig. 10 is as easy to understand as any. We shape the long end of the lever like a pointer. Then the hotter the wire the farther the pointer moves.

If we could put this meter in an electric circuit where we knew one ampere was flowing we could put a numeral "1" opposite where the pointer stood. Then if we could increase the current until there were two amperes flowing through the meter we could mark that position of the pointer "2" and so on. That's the way we would calibrate the meter. After we had done so we would call it an "ammeter" because it measures amperes. Years ago people would have called it an "amperemeter" but no one who is up-to-date would call it so to-day.

If we had a very carefully made ammeter we would send it to the Bureau of Standards to be calibrated. At the Bureau they have a number of meters which they know are correct in their readings. They would put one of their meters and ours into the same circuit so that both carry the same stream of electrons as in Fig. 11. Then whatever the reading was on their meter could be marked opposite the pointer on ours.

Now I want to tell you how the physicists at the Bureau know what is an ampere. Several years ago there was a meeting or congress of physicists and electrical engineers from all over the world who discussed what they thought should be the unit in which to measure current. They decided just what they would call an ampere and then all the countries from which they came passed laws saying that an ampere should be what these scientists had recommended. To-day, therefore, an ampere is defined by law.

To tell when an ampere of current is flowing requires the use of two silver plates and a solution of silver nitrate. Silver nitrate has molecules made up of one atom of silver combined with a group of atoms called "nitrate." You remember that the molecule of copper sulphate, discussed in our third letter, was formed by a copper atom and a group called sulphate. Nitrate is another group something like sulphate for it has oxygen atoms in it, but it has three instead of four, and instead of a sulphur atom there is an atom of nitrogen.

When silver nitrate molecules go into solution they break up into ions just as copper sulphate does. One ion is a silver atom which has lost one electron. This electron was stolen from it by the nitrate part of the molecule when they dissociated. The nitrate ion, therefore, is formed by a nitrogen atom, three oxygen atoms, and one extra electron.

If we put two plates of silver into such a solution nothing will happen until we connect a battery to the plates. Then the battery takes electrons away from one plate and gives electrons to the other. Some of the atoms in the plate which the battery is robbing of electrons are just like the silver ions which are moving around in the solution. That's why they can go out into the solution and play with the nitrate ions each of which has an extra electron which it stole from some silver atom. But the moment silver ions leave their plate we have more silver ions in the solution than we do sulphate ions.

The only thing that can happen is for some of the silver ions to get out of the solution. They aren't going back to the positive silver plate from which they just came. They go on toward the negative plate where the battery is sending an electron for every one which it takes away from the positive plate. There start off towards the negative plate, not only the ions which just came from the positive plate, but all the ions that are in the solution. The first one to arrive gets an electron but it can't take it away from the silver plate. And why should it? As soon as it has got this electron it is again a normal silver atom. So it stays with the other atoms in the silver plate. That's what happens right along. For every atom which is lost from the positive plate there is one added to the negative plate. The silver of the positive plate gradually wastes away and the negative plate gradually gets an extra coating of silver.

Every time the battery takes an electron away from the positive plate and gives it to the negative plate there is added to the negative plate an atom of silver. If the negative plate is weighed before the battery is connected and again after the battery is disconnected we can tell how much silver has been added to it. Suppose the current has been perfectly steady, that is, the same number of electrons streaming through the circuit each second. Then if we know how long the current has been running we can tell how much silver has been deposited each second.

The law says that if silver is being deposited at the rate of 0.001118 gram each second then the current is one ampere. That's a small amount of silver, only about a thousandth part of a gram, and you know that it takes 28.35 grams to make an ounce. It's a very small amount of silver but it's an enormous number of atoms. How many? Six billion billion, of course, for there is deposited one atom for each electron in the stream.

In my next letter I'll tell you how we measure the pull which batteries can give to electrons, and then we shall be ready to go on with more about the audion.



(This letter may be omitted on the first reading.)


I trust you have a fairly good idea that an ampere means a stream of electrons at a certain definite rate and hence that a current of say 3 amperes means a stream with three times as many electrons passing along each second.

In the third and fourth letters you found out why a battery drives electrons around a conducting circuit. You also found that there are several different kinds of batteries. Batteries differ in their abilities to drive electrons and it is therefore convenient to have some way of comparing them. We do this by measuring the electron-moving-force or "electromotive force" which each battery can exert. To express electromotive force and give the results of our measurements we must have some unit. The unit we use is called the "volt."

The volt is defined by law and is based on the suggestions of the same body of scientists who recommended the ampere of our last letter. They defined it by telling how to make a particular kind of battery and then saying that this battery had an electromotive force of a certain number of volts. One can buy such standard batteries, or standard cells as they are called, or he can make them for himself. To be sure that they are just right he can then send them to the Bureau of Standards and have them compared with the standard cells which the Bureau has.

I don't propose to tell you much about standard cells for you won't have to use them until you come to study physics in real earnest. They are not good for ordinary purposes because the moment they go to work driving electrons the conditions inside them change so their electromotive force is changed. They are delicate little affairs and are useful only as standards with which to compare other batteries. And even as standard batteries they must be used in such a way that they are not required to drive any electrons.

Let's see how it can be done. Suppose two boys sit opposite each other on the floor and brace their feet together. Then with their hands they take hold of a stick and pull in opposite directions. If both have the same stick-motive-force the stick will not move.

Now suppose we connect the negative feet—I mean negative terminals—of two batteries together as in Fig. 12. Then we connect their positive terminals together by a wire. In the wire there will be lots of free electrons ready to go to the positive plate of the battery which pulls the harder. If the batteries are equal in electromotive force these electrons will stay right where they are. There will be no stream of electrons and yet we'll be using one of the batteries to compare with the other.

That is all right, you think, but what are we to do when the batteries are not just equal in e. m. f.? (e. m. f. is code for electromotive force). I'll tell you, because the telling includes some other ideas which will be valuable in your later reading.

Suppose we take batteries which aren't going to be injured by being made to work—storage batteries will do nicely—and connect them in series as in Fig. 13. When batteries are in series they act like a single stronger battery, one whose e. m. f. is the sum of the e. m. f.'s of the separate batteries. Connect these batteries to a long fine wire as in Fig. 14. There is a stream of electrons along this wire. Next connect the negative terminal of the standard cell to the negative terminal of the storage batteries, that is, brace their feet against each other. Then connect a wire to the positive terminal of the standard cell. This wire acts just like a long arm sticking out from the positive plate of this cell.

Touch the end of the wire, which is p of Fig. 14, to some point as a on the fine wire. Now what do we have? Right at a, of course, there are some free electrons and they hear the calls of both batteries. If the standard battery, S of the figure, calls the stronger they go to it. In that case move the end p nearer the positive plate of the battery B, so that it will have a chance to exert a stronger pull. Suppose we try at c and find the battery B is there the stronger. Then we can move back to some point, say b, where the pulls are equal.

To make a test like this we put a sensitive current-measuring instrument in the wire which leads from the positive terminal of the standard cell. We also use a long fine wire so that there can never be much of an electron stream anyway. When the pulls are equal there will be no current through this instrument.

As soon as we find out where the proper setting is we can replace S by some other battery, say X, which we wish to compare with S. We find the setting for that battery in the same way as we just did for S. Suppose it is at d in Fig. 14 while the setting for S was at b. We can see at once that X is stronger than S. The question, however, is how much stronger.

Perhaps it would be better to try to answer this question by talking about e. m. f.'s. It isn't fair to speak only of the positive plate which calls, we must speak also of the negative plate which is shooing electrons away from itself. The idea of e. m. f. takes care of both these actions. The steady stream of electrons in the fine wire is due to the e. m. f. of the battery B, that is to the pull of the positive terminal and the shove of the negative.

If the wire is uniform, that is the same throughout its length, then each inch of it requires just as much e. m. f. as any other inch. Two inches require twice the e. m. f. which one inch requires. We know how much e. m. f. it takes to keep the electron stream going in the part of the wire from n to b. It takes just the e. m. f. of the standard cell, S, because when that had its feet braced at n it pulled just as hard at b as did the big battery B.

Suppose the distance n to d (usually written nd) is twice as great as that from n to b (nb). That means that battery X has twice the e. m. f. of battery S. You remember that X could exert the same force through the length of wire nd, as could the large battery. That is twice what cell S can do. Therefore if we know how many volts to call the e. m. f. of the standard cell we can say that X has an e. m. f. of twice as many volts.

If we measured dry batteries this way we should find that they each had an e. m. f. of about 1.46 volts. A storage battery would be found to have about 2.4 volts when fully charged and perhaps as low as 2.1 volts when we had run it for a while.

That is the way in which to compare batteries and to measure their e. m. f.'s, but you see it takes a lot of time. It is easier to use a "voltmeter" which is an instrument for measuring e. m. f.'s. Here is how one could be made.

First there is made a current-measuring instrument which is quite sensitive, so that its pointer will show a deflection when only a very small stream of electrons is passing through the instrument. We could make one in the same way as we made the ammeter of the last letter but there are other better ways of which I'll tell you later. Then we connect a good deal of fine wire in series with the instrument for a reason which I'll tell you in a minute. The next and last step is to calibrate.

We know how many volts of e. m. f. are required to keep going the electron stream between n and b—we know that from the e. m. f. of our standard cell. Suppose then that we connect this new instrument, which we have just made, to the wire at n and b as in Fig. 15. Some of the electrons at n which are so anxious to get away from the negative plate of battery B can now travel as far as b through the wire of the new instrument. They do so and the pointer swings around to some new position. Opposite that we mark the number of volts which the standard battery told us there was between n and b.

If we move the end of the wire from b to d the pointer will take a new position. Opposite this we mark twice the number of volts of the standard cell. We can run it to a point e where the distance ne is one-half nb, and mark our scale with half the number of volts of the standard cell, and so on for other positions along the wire. That's the way we calibrate a sensitive current-measuring instrument (with its added wire, of course) so that it will read volts. It is now a voltmeter.

If we connect a voltmeter to the battery X as in Fig. 16 the pointer will tell us the number of volts in the e. m. f. of X, for the pointer will take the same position as it did when the voltmeter was connected between n and d.

There is only one thing to watch out for in all this. We must be careful that the voltmeter is so made that it won't offer too easy a path for electrons to follow. We only want to find how hard a battery can pull an electron, for that is what we mean by e. m. f. Of course, we must let a small stream of electrons flow through the voltmeter so as to make the pointer move. That is why voltmeters of this kind are made out of a long piece of fine wire or else have a coil of fine wire in series with the current-measuring part. The fine wire makes a long and narrow path for the electrons and so there can be only a small stream. Usually we describe this condition by saying that a voltmeter has a high resistance.

Fine wires offer more resistance to electron streams than do heavy wires of the same length. If a wire is the same diameter all along, the longer the length of it which we use the greater is the resistance which is offered to an electron stream.

You will need to know how to describe the resistance of a wire or of any part of an electric circuit. To do so you tell how many "ohms" of resistance it has. The ohm is the unit in which we measure the resistance of a circuit to an electron stream.

I can show you what an ohm is if I tell you a simple way to measure a resistance. Suppose you have a wire or coil of wire and want to know its resistance. Connect it in series with a battery and an ammeter as shown in Fig. 17. The same electron stream passes through all parts of this circuit and the ammeter tells us what this stream is in amperes. Now connect a voltmeter to the two ends of the coil as shown in the figure. The voltmeter tells in volts how much e. m. f. is being applied to force the current through the coil. Divide the number of volts by the number of amperes and the quotient (answer) is the number of ohms of resistance in the coil.

Suppose the ammeter shows a current of one ampere and the voltmeter an e. m. f. of one volt. Then dividing 1 by 1 gives 1. That means that the coil has a resistance of one ohm. It also means one ohm is such a resistance that one volt will send through it a current of one ampere. You can get lots of meaning out of this. For example, it means also that one volt will send a current of one ampere through a resistance of one ohm.

How many ohms would the coil have if it took 5 volts to send 2 amperes through it. Solution: Divide 5 by 2 and you get 2.5. Therefore the coil would have a resistance of 2.5 ohms.

Try another. If a coil of resistance three ohms is carrying two amperes what is the voltage across the terminals of the coil? For 1 ohm it would take 1 volt to give a current of 1 ampere, wouldn't it? For 3 ohms it takes three times as much to give one ampere. To give twice this current would take twice 3 volts. That is, 2 amperes in 3 ohms requires 2x3 volts.

Here's one for you to try by yourself. If an e. m. f. of 8 volts is sending current through a resistance of 2 ohms, how much current is flowing? Notice that I told the number of ohms and the number of volts, what are you going to tell? Don't tell just the number; tell how many and what.




Although there is much in Letters 7 and 8 which it is well to learn and to think about, there are only three of the ideas which you must have firmly grasped to get the most out of this letter which I am now going to write you about the audion.

First: Electric currents are streams of electrons. We measure currents in amperes. To measure a current we may connect into the circuit an ammeter.

Second: Electrons move in a circuit when there is an electron-moving-force, that is an electromotive force or e. m. f. We measure e. m. f.'s in volts. To measure an e. m. f. we connect a voltmeter to the two points between which the e. m. f. is active.

Third: What current any particular e. m. f. will cause depends upon the circuit in which it is active. Circuits differ in the resistance which they offer to e. m. f.'s. For any particular e. m. f. (that is for any given e. m. f.) the resulting current will be smaller the greater the resistance of the circuit. We measure resistance in ohms. To measure it we find the quotient of the number of volts applied to the circuit by the number of amperes which flow.

In my sixth letter I told you something of how the audion works. It would be worth while to read again that letter. You remember that the current in the plate circuit can be controlled by the e. m. f. which is applied to the grid circuit. There is a relationship between the plate current and the grid voltage which is peculiar or characteristic to the tube. So we call such a relationship "a characteristic." Let us see how it may be found and what it will be.

Connect an ammeter in the plate- or B-circuit, of the tube so as to measure the plate-circuit current. You will find that almost all books use the letter "I" to stand for current. The reason is that scientists used to speak of the "intensity of an electric current" so that "I" really stands for intensity. We use I to stand for something more than the word "current." It is our symbol for whatever an ammeter would read, that is for the amount of current.

Another convenience in symbols is this: We shall frequently want to speak of the currents in several different circuits. It saves time to use another letter along with the letter I to show the circuit to which we refer. For example, we are going to talk about the current in the B-circuit of the audion, so we call that current I{B}. We write the letter B below the line on which I stands. That is why we say the B is subscript, meaning "written below." When you are reading to yourself be sure to read I{B} as "eye-bee" or else as "eye-subscript-bee." I{B} therefore will stand for the number of amperes in the plate circuit of the audion. In the same way I{a} would stand for the current in the filament circuit.

We are going to talk about e. m. f.'s also. The letter "_E_" stands for the number of volts of e. m. f. in a circuit. In the filament circuit the battery has _E_{A}_ volts. In the plate circuit the e. m. f. is _E_{B}_ volts. If we put a battery in the grid circuit we can let _E_{C}_ represent the number of volts applied to the grid-filament or C-circuit.

The characteristic relation which we are after is one between grid voltage, that is E{C}, and plate current, that is I{B}. So we call it the E{C}I{B} characteristic. The dash between the letters is not a subtraction sign but merely a dash to separate the letters. Now we'll find the "ee-see-eye-bee" characteristic.

Connect some small dry cells in series for use in the grid circuit. Then connect the filament to the middle cell as in Fig. 19. Take the wire which comes from the grid and put a battery clip on it, then you can connect the grid anywhere you want along this series of batteries. See Fig. 18. In the figure this movable clip is represented by an arrow head. You can see that if it is at a the battery will make the grid positive. If it is moved to b the grid will be more positive. On the other hand if the clip is at o there will be no e. m. f. applied to the grid. If it is at c the grid will be made negative.

Between grid and filament there is placed a voltmeter which will tell how much e. m. f. is applied to the grid, that is, tell the value of _E_{C}_, for any position whatever of the clip.

We shall start with the filament heated to a deep red. The manufacturers of the audion tell the purchaser what current should flow through the filament so that there will be the proper emission of electrons. There are easy ways of finding out for one's self but we shall not stop to describe them. The makers also tell how many volts to apply to the plate, that is what value _E_{B}_ should have. We could find this out also for ourselves but we shall not stop to do so.

Now we set the battery clip so that there is no voltage applied to the grid; that is, we start with E{C} equal to zero. Then we read the ammeter in the plate circuit to find the value of I{B} which corresponds to this condition of the grid.

Next we move the clip so as to make the grid as positive as one battery will make it, that is we move the clip to a in Fig. 19. We now have a different value of E{C} and will find a different value of I{B} when we read the ammeter. Next move the clip to apply two batteries to the grid. We get a new pair of values for E{C} and I{B}, getting E{C} from the voltmeter and I{B} from the ammeter. As we continue in this way, increasing E{C}, we find that the current I{B} increases for a while and then after we have reached a certain value of E{C} the current I{B} stops increasing. Adding more batteries and making the grid more positive doesn't have any effect on the plate current.

Before I tell you why this happens I want to show you how to make a picture of the pairs of values of E{C} and I{B} which we have been reading on the voltmeter and ammeter.

Imagine a city where all the streets are at right angles and the north and south streets are called streets and numbered while the east and west thorofares are called avenues. I'll draw the map as in Fig. 20. Right through the center of the city goes Main Street. But the people who laid out the roads were mathematicians and instead of calling it Main Street they called it "Zero Street." The first street east of Zero St. we should have called "East First Street" but they called it "Positive 1 St." and the next beyond "Positive 2 St.," and so on. West of the main street they called the first street "Negative 1 St." and so on.

When they came to name the avenues they were just as precise and mathematical. They called the main avenue "Zero Ave." and those north of it "Positive 1 Ave.," "Positive 2 Ave." and so on. Of course, the avenues south of Zero Ave. they called Negative.

The Town Council went almost crazy on the subject of numbering; they numbered everything. The silent policeman which stood at the corner of "Positive 2 St." and "Positive 1 Ave." was marked that way. Half way between Positive 2 St. and Positive 3 St. there was a garage which set back about two-tenths of a block from Positive 1 Ave. The Council numbered it and called it "Positive 2.5 St. and Positive 1.2 Ave." Most of the people spoke of it as "Plus 2.5 St. and Plus 1.2 Ave."

Sometime later there was an election in the city and a new Council was elected. The members were mostly young electricians and the new Highway Commissioner was a radio enthusiast. At the first meeting the Council changed the names of all the avenues to "Mil-amperes"[3] and of all the streets to "Volts."

Then the Highway Commissioner who had just been taking a set of voltmeter and ammeter readings on an audion moved that there should be a new road known as "Audion Characteristic." He said the road should pass through the following points:

Zero Volt and Plus 1.0 Mil-ampere Plus 2.0 Volts and Plus 1.7 Mil-amperes Plus 4.0 Volts and Plus 2.6 Mil-amperes Plus 6.0 Volts and Plus 3.4 Mil-amperes Plus 8.0 Volts and Plus 4.3 Mil-amperes

And so on. Fig. 21 shows the new road.

One member of the Council jumped up and said "But what if the grid is made negative?" The Commissioner had forgotten to see what happened so he went home to take more readings.

He shifted the battery clip along, starting at c of Fig. 22. At the next meeting of the Council he brought in the following list of readings and hence of points on his proposed road.

Minus 1.0 Volts and Plus 0.6 Mil-ampere " 2.0 " " " 0.4 " " " 3.0 " " " 0.2 " " " 4.0 " " " 0.1 " " " 5.0 " " " 0.0 " "

Then he showed the other members of the Council on the map of Fig. 23 how the Audion Characteristic would look.

There was considerable discussion after that and it appeared that different designs and makes of audions would have different characteristic curves. They all had the same general form of curve but they would pass through different sets of points depending upon the design and upon the B-battery voltage. It was several meetings later, however, before they found out what effects were due to the form of the curve. Right after this they found that they could get much better results with their radio sets.

Now look at the audion characteristic. Making the grid positive, that is going on the positive side of the zero volts in our map, makes the plate current larger. You remember that I told you in Letter 6 how the grid, when positive, helped call electrons away from the filament and so made a larger stream of electrons in the plate circuit. The grid calls electrons away from the filament. It can't call them out of it; they have to come out themselves as I explained to you in the fifth letter.

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