The Standard Electrical Dictionary - A Popular Dictionary of Words and Terms Used in the Practice - of Electrical Engineering
by T. O'Conor Slone
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Generator, Secondary. A secondary or storage battery. (See Battery, Secondary.)

German Silver. An alloy of copper, 2 parts, nickel, 1 part, and zinc, 1 part. Owing to its high resistance and moderate cost and small variation in resistance with change of temperature, it is much used for resistances. From Dr. Mathiessen's experiment the following constants are deduced in legal ohms: Relative Resistance (Silver = 1), 13.92 Specific Resistance at 0 C. (32F.), 20.93 microhms. Resistance of a wire, (a) 1 foot long, weighing 1 grain, 2.622 ohms. 1 foot long, 1/1000 inch thick, 125.91 " 1 meter long, weighing 1 gram, 1.830 " 1 meter long, 1 millimeter thick, 0.2666 " Resistance of a 1 inch cube at 0C. (32 F.), 8.240 microhms.

Approximate percentage increase of resistance per 1 C. (1.8 F.) at about 20 C. (68 F.), 0.044 per cent.

Gilding, Electro-. The deposition of gold by an electric current, or electrolytically in the electroplating bath.

Gilding Metal. A special kind of brass, with a high percentage of copper, used to make objects which are to be gilded by electrolysis.


Gimbals. A suspension used for ships' compasses and sometimes for other apparatus. It consists of a ring held by two journals, so as to bc free to swing in one plane. The compass is swung upon this ring, being placed concentrically therewith. Its journals are at right angles to those of the ring. This gives a universal joint by which the compass, weighted below its line of support, is always kept horizontal.


Glass. A fused mixture of silicates of various oxides. It is of extremely varied composition and its electric constants vary greatly. Many determinations of its specific resistance have been made. For flint glass at 100 C. (212 F.) about (2.06E14) ohms —at 60 C (140 F.) (1.020E15) (Thomas Gray) is given, while another observer (Beetz) gives for glass at ordinary temperatures an immeasurably high resistance. It is therefore a non-conductor of very high order if dry. As a dielectric the specific inductive capacity of different samples of flint glass is given as 6.57—6.85—7.4—10.1 (Hopkinson), thus exceeding all other ordinary dielectrics. The densest glass, other things being equal, has the highest specific inductive capacity.

Gold. A metal, one of the elements; symbol Au. c .; atomic weight, 196.8; equivalent, 65.6; valency, 3; specific gravity 19.5. It is a conductor of electricity.

Annealed. Hard drawn. Relative Resistance (Annealed Silver = 1), 1.369 1.393 Specific Resistance, 2.058 2.094 Resistance of a wire at 0 C. (32F.) (a) 1 foot long, weighing 1 grain, 57.85 58.84 ohms (b) 1 foot long, 1/1000 inch thick, 12.38 12.60 " (c) 1 meter long, weighing 1 gram, .4035 .4104 " (d) 1 meter long, 1 millimeter thick, .02620 .02668 " Resistance of a 1 inch cube at 0 C.(32 F.) .8102 .8247

Approximate increase in resistance per 0 C., (1.8 F) at about 20 C. (68 F.), 0.365 per cent.

Electro-chemical equivalent (Hydrogen = .0105), .6888


Gold Bath. A solution of gold used for depositing the metal in the electroplating process.

A great number of formulae have been devised, of which a few representative ones are given here. COLD BATHS. HOT BATHS. Water, 10,000 10,000 10,000 10,000 5,000 3,000 Potassium Cyanide, 200 — 200 10 — 50 Gold, 100 15 100 10 10 10 Potassium Ferrocyanide, — 200 — — 150 — Potassium Carbonate, — 150 — — 50 — Ammonium Chloride, — 30 — — 20 — Aqua Ammoniae, — — 500 — — — Sodium Phosphate, — — — 600 — — Sodium Bisulphite, — — — 100 — —


In the baths the gold is added in the form of neutral chloride, Auric chloride (Au Cl6).

Gold Stripping Bath. A bath for removing gold from plated articles without dissolving the base in order to save the precious metal. A bath of 10 parts of potassium cyanide and 100 parts of water may be used, the articles to be stripped being immersed therein as the anode of an active circuit. If the gilding is on a silver or copper basis, or on an alloy of these metals the same solution attacks the base and dissolves it, which is objectionable. For silver articles it is enough to heat to cherry red and throw into dilute sulphuric acid. The gold scales off in metallic spangles. For copper articles, a mixture of 10 volumes concentrated sulphuric acid, 1 volume nitric acid, and 2 volumes hydrochloric acid may be used by immersion only, or with a battery. The sulphuric acid in such large excess is supposed to protect the copper. For copper articles concentrated sulphuric acid alone with the battery may be used. This does not sensibly attack the copper if it is not allowed to become diluted. Even the dampness of the air may act to dilute it.

Graduator. Apparatus for enabling the same line to be used for telegraph signals and telephoning.

One type consists in coils with iron cores or simply electromagnets. These act to retard the current in reaching its full power and also prolong it. This gives a graduated effect to the signals, so that the telephone diaphragm is not audibly affected by the impulses.

The telephoning current is so slight and so rapid in its characteristic changes that it is without effect upon the ordinary telegraph.


Gram. The unit of weight in the metric system; accepted as the unit of mass in the absolute of C. G. S. system of units. It is the one-thousandth part of mass of a standard weight preserved under proper conditions in Paris, and supposed to be the mass of a cubic decimeter of distilled water at the temperature of the maximum density of water. The standard is the kilogram; the temperature is 3.9 C. (39 F.). The standard kilogram is found to be not exactly the weight of a cubic decimeter of water, the latter weighing 1.000013 kilogram.

If therefore the defined gram on the water basis is taken as the unit it varies very slightly from the accepted gram.

1 gram is equal to 15.43234874 grains. (Prof. W. H. Miller.)

Gram-atom. The number of grams of an element equal numerically to the atomic weight, as 16 grams of oxygen, 1 gram of hydrogen, 35.5 grams of chlorine; all which might be expressed as gram-atoms of oxygen, hydrogen and chlorine respectively.

The gram-atom approximately expresses the number of gram-calories required to heat one gram of the substance 1 C. (1.8 F.). This is in virtue of Dulong and Petit's discovery that the atomic weight of an element multiplied by its specific heat gives approximately a constant for all elements.

[Transcriber's note: A gram-atom is the mass, in grams, of one mole of atoms in a monatomic element. A mole consists of Avogadro's number of atoms, approximately 6.02214E23.]

Gram-molecule. The number of grams of a substance equal numerically to its molecular weight.

Graphite. Carbon; one of three allotropic modifications of this element. It occurs in nature as a mineral.

It is used as a lubricant for machinery; for commutator brushes; for making surfaces to be plated conductive, and for mixing with manganese binoxide in Leclanch cells.

Gravitation. A natural force which causes all masses of matter to attract each other. Its cause is unknown; it is often supposed to be due to the luminiferous ether.

[Transcriber's note: Einstein's explanation of gravity, General Relativity and the curvature of space-time, came 23 years later, 1915.]


Gravity, Acceleration of. The velocity imparted to a body in one second by the action of gravitation at any standard point upon the earth's surface in a vacuum. This will vary at different places, owing principally to the variation in centrifugal force due to the earth's rotation. For standard valuation it must be reduced to sea level. The following are examples of its variation:

Equator, 978.1028 centimeters per second Paris, 980.94 " Greenwich 981.I7 " Edinburgh, 981.54 " Pole (N. or S.), 983.1084 (theoretical) "

As round numbers for approximate calculations 981 centimeters or 32.2 feet may be employed.

[Transcriber's note: The acceleration of gravity at the equator is also reduced by the increased distance from the center of the earth (equatorial bulge). Increased altitude reduces gravity. Reduced air density at altitude reduces buoyancy and increases apparent weight. Local variations of rock density affects gravity.]

Gravity, Control. Control by weight. In some ammeters and voltmeters gravity is the controlling force.

Grid. A lead plate perforated or ridged for use in a storage battery as the supporter of the active materials and in part as contributing thereto from its own substance.

Ground. The contact of a conductor of an electric circuit with the earth, permitting the escape of current if another ground exists.

Ground-wire. A metaphorical term applied to the earth when used as a return circuit.


Grove's Gas Battery. A voltaic battery depending for its action on the oxidation of hydrogen instead of the oxidation of zinc. Its action is more particularly described under Battery, Gas. In the cut B, B1 * * * are the terminals of the positive or hydrogen electrodes, marked H, and A, Al * * * are the terminals of the negative or oxygen electrodes marked O, while M, M1 * * * is dilute sulphuric acid.


Guard Ring. An annular horizontal surface surrounding the balanced disc in the absolute electrometer. (See Electrometer, Absolute.)

Guard Tube. A metal tube surrounding a dry pile used with a quadrant electrometer, or other electrometers of that type. It prevents the capacity of the lower brass end of the pile (which brass end closes the glass tube containing the discs) from momentary change by approach of some conductor connected to the earth. There are other guard tubes also.

Gun, Electro-magnetic. An electro-magnet with tubular core. If, when it is excited a piece of an iron rod is pushed into the central aperture of the core and is released, the magnetic circle will try to complete itself by pushing the rod out so that it can thus be discharged, as if from a popgun.

Synonym—Electric Popgun.


Gutta Percha. The hardened milky juice of a tree, the Isonandra gutta, growing in Malacca and other parts of the Eastern Archipelago. It is much used as an insulator or constituent of insulators.

Resistance after several minutes electrification per 1 centimeter cube at 54 C. (75 F.), 4.50E14 ohms.

The specific resistance varies—from 2.5E13 to 5.0E14 ohms. A usual specification is 2.0E14 ohms. The influence of temperature on its resistance is given in Clark & Bright's empirical formula, R = R0 at, in which R is the resistance at temperature t C—Ro the resistance at 0 C (32 F), a is the coefficient .8944.

The resistance increases with the time of passage of the current, the variation being less the higher the temperature.


Time of Relative Resistance Relative Resistance Electrification. at 0 C (32 F.) at 24 C (75 F.) 1 minute 100 5.51 2 " 127.9 6. 5 " 163.1 6.66 10 " 190.9 6.94 20 " 230.8 7.38 30 " 250.6 7.44 60 " 290.4 7.6 90 " 318.3 7.66

In cable testing one minute is generally taken as the time of electrification.

Pressure increases the resistance by the formula Rp=R (1+ .00327 P) in which Rp is the resistance at pressure p—R resistance at atmospheric pressure—p pressure in atmospheres. Thus in the ocean at a depth of 4,000 meters (2.4855 miles), the resistance is more than doubled. The longer the pressure is applied, the greater is the resistance.

The specific inductive capacity of gutta percha is 4.2.

Good gutta percha should not break when struck with a hammer, should recover its shape slowly, and it should support much more than 300 times its own weight.

Gyrostatic Action of Armatures. Owing to gyrostatic action a rotating armature resists any change of direction of its axis. On ships and in railway motors which have to turn curves this action occurs. A 148 lb. armature running at 1,300 revolutions per minute may press with 30 lbs. on each journal as the ship rolls through an angle of 20 in 16 seconds.

H. (a) The symbol for the horizontal component of the earth's magnetization.

(b) The symbol for the intensity of a magnetizing force or field. The symbol H, as it is generally used, may mean either the number of dynes which act upon a unit pole, or the number of lines of force per centimeter.

(c) The symbol for the unit of self-induction.

Hair, Removal of, by Electrolysis. A method of depilation by destruction of individual hair follicles by electrolysis.

A fine platinum electrode is thrust into a hair follicle. It is the negative electrode. The positive electrode is in contact with the body of the person under treatment; it is often a sponge electrode simply held in his hand. A current of two to four milliamperes from an E. M. F. of 15 to 20 volts, is passed. This destroys the follicle, the hair is removed and never grows again. A gradual increase of current is advised for the face. As only one hair is removed at once, but a small number are taken out at a sitting.


Haldat's Figures. With a pole of a strong bar magnet, used like a pencil, imaginary figures are drawn upon a hard steel plate, such as a saw-blade. The pattern is gone over several times. By dusting iron filings on a sheet of paper laid over the steel plate, while horizontal, very complicated magnetic figures are produced.

Hall's Experiment. A cross of thin metal, such as gold leaf, is secured upon a pane of glass. To two opposite arms a battery is connected in circuit with them. To the other two arms a galvanometer is connected in circuit. If the cross is put into a field of force whose lines are perpendicular thereto, the galvanometer will disclose a constant current. The current is pushed, as it were, into the galvanometer circuit. Other metals have been used with similar results. They must be thin or the experiment fails. If the arm receiving the battery current is horizontal, and if it flows from left to right, and if the lines of force go from downward through the cross, the current in the galvanometer circuit will flow from the observer through the other arms of the cross, if the cross is of gold, silver, platinum or tin, and the reverse if of iron. The experiment has indicated a possible way of reaching the velocity of electricity in absolute measure.

Hall Effect. The effect observed in Hall's experiment, q. v.

Hall Effect, Real. A transverse electro-motive force in a conductor through which a current is passing produced by a magnetic field.

Hall Effect, Spurious. A spurious electro-motive force produced in a conductor, through which a current is passing by changes in conductivity of the conductor brought about by a magnetic field.

Hanger Board. A board containing two terminals, a suspending hook, and a switch, so that an arc lamp can be introduced into a circuit thereby, or can be removed as desired.

Harmonic Receiver. A receiver containing a vibrating reed, acted on by an electro-magnet. Such a reed answers only to impulses tuned to its own pitch. If such are received from the magnet it will vibrate. Impulses not in tune with it will not affect it. (See Telegraph, Harmonic.)

Head Bath, Electric. A fanciful name for an electro-medical treatment of the head. The patient is insulated by an insulating stool or otherwise. His person is connected with one terminal of an influence machine. An insulated metallic circle, with points of metal projecting inward or downward, is placed about the head. The circle is connected with the other pole of the machine. On working it a silent or brush discharge with air convection streams occurs between the patient's head and the circle of points.


Head-light, Electric. An electric head-light for locomotives has been experimented with. It includes the parabolic reflection of the regular light with an arc-lamp in place of the oil lamp. An incandescent lamp may be used in the same place, but has no great advantage over oil as regards illuminating power.

Heat. A form of kinetic energy, due to a confused oscillatory movement of the molecules of a body. Heat is not motion, as a heated body does not change its place; it is not momentum, but it is the energy of motion. If the quantity of molecular motion is doubled the momentum of the molecules is also doubled, but the molecular mechanical energy or heat is quadrupled.

As a form of energy it is measured by thermal units. The calorie is the most important, and unfortunately the same term applies to two units, the gram-degree C. and the kilogram-degree C. (See Calorie.) Calories are determined by a calorimeter, q. v.

Independent of quantity of heat a body may be hotter or colder. Thermometers are used to determine its temperature.

Heat is transmitted by conduction, a body conducting it slowly for some distance through its own substance. Bodies vary greatly in their conductivity for heat. It is also transmitted by convection of gases or liquids, when the heated molecules traveling through the mass impart their heat to other parts. Finally it is transmitted by ether waves with probably the speed of light. This mode of transmission and the phenomena of it were attributed to radiant heat. As a scientific term this is now dropped by many scientists. This practice very properly restricts the term "heat" to kinetic molecular motion.

The mechanical equivalent of heat is the number of units of work which the energy of one unit quantity of heat represents. (See Equivalents, Mechanical and Physical.)

Heat, Atomic. The product of the specific heat of an element by its atomic weight. The product is approximately the same for all the elements, and varies as determined between 5.39 and 6.87. The variations are by some attributed principally to imperfection of the work in determining them. The atomic heat represents the number of gram calories required to raise the temperature of a gram atom (a number of grams equal numerically to the atomic weight) one degree centigrade.


Heat, Electric. This term has been given to the heat produced by the passage of a current of electricity through a conductor. It is really electrically produced heat, the above term being a misnomer.

The rise of temperature produced in a cylindrical conductor by a current depends upon the diameter of the conductor and on the current. The length of the wire has only the indirect connection that the current will depend upon the resistance and consequently upon its length.

The quantity of heat produced in a conductor by a current is in gram-degree C. units equal to the product of the current, by the electro-motive force or potential difference maintained between the ends of the wire, by .24.

The cube of the diameter of a wire for a given rise of temperature produced in such conductor by a current is equal approximately to the product of the square of the current, by the specific resistance (q. v.) of the material of the conductor, by .000391, the whole divided by the desired temperature in centigrade units.

Heat, Electrical Convection of. A term applied to the phenomena included under the Thomson effect, q. v., the unequal or differential heating effect produced by a current of electricity in conductors whose different parts are maintained at different temperatures.

Heater, Electric. An apparatus for converting electrical energy into thermal energy.

An incandescent lamp represents the principle, and in the Edison meter has been used as such to maintain the temperature of the solutions. Heaters for warming water and other purposes have been constructed, utilizing conductors heated by the passage of the current as a source of heat. (See also Heating Magnet.)

Heating Error. In voltmeters the error due to alteration of resistance of the coil by heating. If too strong a current is sent through the instrument, the coils become heated and their resistance increased. They then do not pass as much current as they should for the potential difference to which they may be exposed. Their readings then will be too low. One way of avoiding the trouble is to have a key in circuit, and to pass only an instantaneous or very brief current through the instrument and thus get the reading before the coils have time to heat.

The heating error does not exist for ammeters, as they are constructed to receive the entire current, and any heating "error" within their range is allowed for in the dividing of the scale.

Heating Magnet. An electro-magnet designed to be heated by Foucault currents induced in its core by varying currents in the windings. It has been proposed as a source of artificial heat, a species of electric heating apparatus for warming water, or other purposes.


Heat, Irreversible. The heat produced by an electric current in a conductor of identical qualities and temperature throughout. Such heat is the same whatever the direction of the current. The heating effect is irreversible because of the absence of the Thomson effect, q. v.) or Peltier effect, q. v.

Heat, Mechanical Equivalent of. The mechanical energy corresponding to a given quantity of heat energy. Mechanical energy is generally represented by some unit of weight and height, such as the foot-pound; and heat energy is represented by a given weight of water heated a given amount, such as a pound-degree centigrade. Joule's equivalent is usually accepted; it states that 772.55 foot pounds of mechanical energy are equivalent to 1 pound-degree F. (one pound avds. of water raised in temperature one degree Fahrenheit). Other equivalencies have also been deduced.

Heat, Molecular. The product of a specific heat of the compound by its molecular weight. It is approximately equal to the sum of the atomic heats of its constituent elements.

The molecular heat represents the number of gram calories required to raise the temperature of a gram-molecule (a number of grams equal numerically to the molecular weight) one degree centigrade.

The molecular heat is approximately equal for all substances.

Heat, Specific. The capacity of a body for heat; a coefficient representing the relative quantity of heat required to raise the temperature of an identical weight of a given body a defined and identical amount.

The standard of comparison is water; its specific heat is taken as unity. The specific heats by weight of other substances are less than unity. The specific heat varies with the temperature. Thus the specific heat of water is more strictly 1+.00015 t C.

Specific heat is greater when a substance is in the liquid than when it is in the solid state. Thus the specific heat of ice is 0.489; less than half that of water. It differs with the allotropic modifications of bodies; the specific heat of graphite is .202; of diamond, .147.

The product of the specific heat by the atomic weight of elements gives a figure approximately the same. A similar law applies in the case of molecules. (See Heat, Atomic-Heat, Molecular.)

The true specific heat of a substance should be separated from the heat expended in expanding a body against molecular and atomic forces, and against the atmospheric pressure. So far this separation has not been possible to introduce in any calculations.


Heat, Specific, of Electricity. A proposed term to account for the heat absorbed or given out in unhomogeneous conductors, by the Thomson effect, or Peltier effect (see Effect, Thomson—Effect, Peltier.) If a current of electricity be assumed to exist, then under the action of these effects it may be regarded as absorbing or giving out so many coulombs of heat, and thus establishing a basis for specific heat.

Heat Units. The British unit of heat is the pound degree F—the quantity of heat required to raise the temperature of a pound of water from 32 to 33 F.

The C. G. S. unit is the gram-degree C.; another metric unit is the kilogram-degree C. The latter is the calorie; the former is sometimes called the small calorie or the joule; the latter is sometimes called the large calorie. The term joule is also applied to a quantity of heat equivalent to the energy of a watt-second or volt-coulomb. This is equal to .24l gram degree calorie.

Hecto. A prefix to terms of measurement—meaning one hundred times, as hectometer, one hundred meters.

Heliograph. An apparatus for reflecting flashes of light to a distant observer. By using the Morse telegraph code messages may thus be transmitted long distances. When possible the sun's light is used.

Helix. A coil of wire; properly a coil wound so as to follow the outlines of a screw without overlaying itself.



Henry. The practical unit of electro-magnetic or magnetic inductance. It is equal to 1E9 C. G. S., or absolute units of inductance. As the dimensions of inductance are a length the henry is equal to 1E9 centimeters, or approximately to one quadrant of the earth measured on the meridian.



Hermetically Sealed. Closed absolutely tight. Glass vessels, such as the bulbs of incandescent lamps, are hermetically sealed often by melting the glass together over any opening into their interior.

Heterostatic Method. A method of using the absolute or attracted disc electrometer. (See Electrometer Absolute.) The formula for its idiostatic use, q. v., involves the determination of d, the distance between the suspended and fixed discs. As this is difficult to determine the suspended disc and guard ring may be kept at one potential and the lower fixed disc is then connected successively with the two points whose potential difference is to be determined. Their difference is determined by the difference between d and d', the two distances between the discs. This difference is the distance through which the micrometer screw is moved. The heterostatic formula is:

V' - V = (d' - d)* squareRoot( 8*PI*F / S )

in which V and V' are potentials of the two points; d' and d the two distances between the discs necessary for equilibrium; S the area of the disc and F the force of attraction in dynes. (See Idiostatic Method.)

High Bars of Commutator. Commutator bars, which in the natural wear of the commutator, project beyond the others. The surface then requires turning down, as it should be quite cylindrical.

High Frequency. A term used as a noun or as an adjective to indicate in an alternating current, the production of a very great number of alternations per unit of time—usually expressed as alternations per second.

Hissing. A term applied to a noise sometimes produced by a voltaic arc; probably due to the same cause as frying, q. v.

Hittorf's Solution. A solution used as a resistance. It is a solution of cadmium iodide in amylic alcohol. Ten per cent. of the salt is used. It is contained in a tube with metallic cadmium electrodes. (See Resistance, Hittorf' s.)



Holders. (a) The adjustable clamps for holding the armature brushes of dynamos and motors.

(b) The clamps for holding the carbons of arc lamps.

(c) The clamps for holding safety fuses, q. v.

(d) Holders for Jablochkoff candles and other electric candles. (See Candle Holders.)

(e) A box or block of porcelain for holding safety fuses.

Hood. A tin hood placed over an arc-lamp. Such hoods are often truncated cones in shape, with the small end upwards. They reflect a certain amount of light besides protecting the lamp to some extent from rain.

Horns. The extensions of the pole pieces of a dynamo or motor. (See Following Horns-Leading Horns.)

Synonym—Pole Tips.

Horse Power. A unit of rate of work or activity. There are two horse powers.

The British horse power is equal to 33,000 pounds raised one foot per minute, or 550 foot pounds per second, or 1.0138 metric horse power.

The metric horse power (French) is equal to 75 kilogram-meters, or 542 foot pounds per second, or .986356 British horse power.

H. P. is the abbreviation for horse power. (See Horse Power, Electric.)

Horse Power, Actual. The rate of activity of a machine, as actually developed in condition for use. It is less than the indicated or total horse power, because diminished by the hurtful resistances of friction, and other sources of waste. It is the horse power that can be used in practise, and which in the case of a motor can be taken from the fly-wheel.

Horse Power, Electric. The equivalent of a mechanical horse power in electric units, generally in volt-amperes or watts; 745.943 watts are equivalent to the activity of one British horse power; 735.75 are equivalent to one metric horse power. The number 746 is usually taken in practical calculations to give the equivalency.

[Transcriber's note: Contemporary values are: Mechanical (British) horsepower = 745.6999 Watts; Metric horsepower = 735.49875 Watts]

Horse Power, Indicated. The horse power of an engine as indicated by its steam pressure, length of stroke, and piston area, and vacuum, without making any deduction for friction or hurtful resistances. The steam pressure is in accurate work deduced from indicator diagrams.

Horse Power, Hour. A horse power exerted for one hour, or the equivalent thereof. As the horse power is a unit of activity, the horse power hour is a unit of work or of energy. It is equal to 1,980,000 foot pounds.

H. P. Abbreviation for "horse power."


Hughes' Electro-magnet. A horseshoe electro-magnet with polarized core. It is made by mounting two bobbins of insulated wire on the ends of a permanent horseshoe magnet. It was devised for use in Hughes' printing telegraph, where very quick action is required. The contact lasts only .053 second, 185 letters being transmitted per minute.



Hughes' Induction Balance. An apparatus for determining the presence of a concealed mass of metal. The apparatus is variously connected. The cut shows a representative form; a and a' are two primary coils, each consisting of 100 meters (328 feet) of No. 32 silk covered copper wire (0.009 inch diameter) wound on a boxwood spool ten inches in depth; b and b' are secondary coils. All coils are supposed to be alike. The primary coils are joined in series with a battery of three or four Daniell cells. A microphone m is included in the same circuit. The secondary coils are joined in series with a telephone and in opposition with each other. The clock is used to produce a sound affecting the microphone. If all is exactly balanced there will be no sound produced in the telephone. This balance is brought about by slightly varying the distance of one of the secondaries from the primary, until there is no sound in the telephone. If now a piece of metal is placed within either of the coils, it disturbs the balance and the telephone sounds.


To measure the forces acting a sonometer or audiometer is used. This is shown in the upper part of the cut. Two fixed coils, c and e are mounted at the ends of a graduated bar. A movable coil d is connected in the telephone circuit; c and e by a switch can be connected with the battery and microphone circuit, leaving out the induction balance coils. The ends of the coils c and e, facing each other are of the same polarity. If these coils, c and e, were equal in all respects, no sound would be produced when d was midway between them. But they are so wound that the zero position for d is very near one of them, c.

Assume that a balance has been obtained in the induction balance with the coil d at zero. No sound is heard whether the switch is moved to throw the current into one or the other circuit. A piece of metal placed in one of the balance coils will cause the production of a sound. The current is turned into the sonometer and d is moved until the same sound, as tested by rapid movements of the switch, is heard in both circuits. The displacement of d gives the value of the sound.

A milligram of copper is enough to produce a loud sound. Two coins can be balanced against each other, and by rubbing one of them, or by breathing on one of them, the balance will be disturbed and a sound will be produced.

Prof. Hughes has also dispensed with the audiometer. He has used a strip of zinc tapering from a width of 4 mm. (.16 inch) at one end to a sharp edge or point at the other. The piece to be tested being in place in one coil, the strip is moved across the face of the other until a balance is obtained.

As possible uses the detection of counterfeit coins, the testing of metals for similarity of composition and the location of bullets in the body have been suggested. Care has to be taken that no masses of metal interfere. Thus in tests of the person of a wounded man, the presence of an iron truss, or of metallic bed springs may invalidate all conclusions.

The same principle is carried out in an apparatus in which the parts are arranged like the members of a Wheatstone bridge. One pair of coils is used, which react on each other as primary and secondary coils. One of the coils is in series with a telephone in the member of the bridge corresponding to that containing the galvanometer of the Wheatstone bridge. The latter is more properly termed an induction bridge.

Synonyms—Inductance Bridge—Inductance Balance—Induction Bridge.


Hydro-electric. adj. (a) A current produced by a voltaic couple or the couple itself is sometimes thus characterized or designated as a "hydro-electric current" or a "hydro-electric couple." It distinguishes them from thermo-electric.

(b) Armstrong's steam boiler electric machine (see Hydroelectric Machine) is also termed a hydro-electric machine.

Hydro-electric Machine. An apparatus for generating high potential difference by the escape of steam through proper nozzles.

It consists of a boiler mounted on four glass legs or otherwise insulated. An escape pipe terminates in a series of outlets so shaped as to impede the escape of the steam by forcing it out of the direct course. These jets are lined with hard wood. They are enclosed in or led through a box which is filled with cold water.


This is to partly condense the steam so as to get it into the vesicular state, which is found essential to its action. Dry steam produces no excitation. If the boiler is fired and the steam is permitted to escape under the above conditions the vesicles presumably, or the "steam" is found to be electrified. A collecting comb held against the jet becomes charged and charges any connected surface.


The boiler in the above case is negatively and the escaping "steam" is positively charged. By changing the material of the linings of the jets, or by adding turpentine the sign of the electricity is reversed. If the water contains acid or salts no electricity is produced. The regular hydro-electric machine is due to Sir William Armstrong.

Faraday obtained similar results with moist air currents.

Hydrogen. An element existing under all except the most extreme artificial conditions of pressure and cold as a gas. It is the lightest of known substances. Atomic weight, 1; molecular weight, 2; equivalent, 1; valency, 1; specific gravity, .0691-.0695. (Dumas & Boussingault.)

It is a dielectric of about the same resistance as air. Its specific inductive capacity at atmospheric pressure is: .9997 (Baltzman) .9998 (Ayrton)

Electro-chemical equivalent, .0105 milligram. The above is usually taken as correct. Other values are as follows: .010521 (Kohllrausch) .010415 (Mascart)

The electro-chemical equivalent of any element is obtained by multiplying its equivalent by the electro-chemical equivalent of hydrogen. The value .0105 has been used throughout this book.

Hygrometer. An instrument for determining the moisture in the air. One form consists of a pair of thermometers, one of which has its bulb wrapped in cloth which is kept moist during the observation. The evaporation is more or less rapid according to the dryness or moisture of the air, and as the temperature varies with this evaporation the relative readings of the two thermometers give the basis for calculating the hygrometric state of the air. Another form determines the temperature at which dew is deposited on a silver surface, whence the calculations are made.

Hysteresis, Magnetic. A phenomenon of magnetization of iron. It may be attributed to a sort of internal or molecular friction, causing energy to be absorbed when iron is magnetized. Whenever therefore the polarity or direction of magnetization of a mass of iron is rapidly changed a considerable expenditure of energy is required. It is attributed to the work done in bringing the molecules into the position of polarity.


The electric energy lost by hysteresis may be reduced by vibrations or jarring imparted to the iron, thus virtually substituting mechanical for electrical work.

On account of hysteresis the induced magnetization of a piece of iron or steel for fields of low intensity will depend on the manner in which the material has been already magnetized. Let the intensity of field increase, the magnetization increasing also; then lower the intensity; the substance tends to and does retain some of its magnetism. Then on again strengthening the field it will have something to build on, so that when it attains its former intensity the magnetization will exceed its former value. For a moderate value of intensity of field the magnetization can have many values within certain limits.

Synonym—Hysteresis—Hysteresis, Static—Magnetic Friction.

Hysteresis, Viscous. The gradual increase or creeping up of magnetization when a magnetic force is applied with absolute steadiness to a piece of iron. It may last for half an hour or more and amount to several per cent. of the total magnetization. It is a true magnetic lag.

I. A symbol sometimes used to indicate current intensity. Thus Ohm's law is often expressed I = E/R, meaning current intensity is equal to electro-motive force divided by resistance. C is the more general symbol for current intensity.

Ideoelectrics or Idioelectrics. Bodies which become electric by friction. This was the old definition, the term originating with Gilbert. It was based on a misconception, as insulation is all that is requisite for frictional electrification, metals being thus electrified if held by insulating handles. The term is virtually obsolete; as far as it means anything it means insulating substances such as scaling wax, sulphur, or glass.

Idle Coils. Coils in a dynamo, in which coils no electro-motive force is being generated. This may occur when, as a coil breaks connection with the commutator brush, it enters a region void of lines of magnetic force, or where the lines are tangential to the circle of the armature.

Idiostatic Method. A method of using the absolute or attracted disc electrometer. (See Electrometer, Absolute.) The suspended disc and guard ring are kept at the same potential, which is that of one of the points whose potential difference is to be determined; the lower fixed disc is connected to the other of the points whose potential difference is to be determined. Then we have the formula

V = d * SquareRoot( 8 * PI * F ) / S

in which d is the distance between the discs, V is the difference of potential of the two points, F the force of attraction between the discs in dynes, and S the area of the suspended disc. (See Heterostatic Method.)


Idle Poles. Poles of wire sealed into Crookes' tubes, not used for the discharge connections, but for experimental connections to test the effect of different excitation on the discharge.

Idle Wire. In a dynamo the wire which plays no part in generating electro-motive force. In a Gramme ring the wire on the inside of the ring is idle wire.

Igniter. In arc lamps with fixed parallel carbons of the Jablochkoff type (see Candle, Jablochkoff) a strip of carbon connects the ends of the carbons in the unused candle. This is necessary to start the current. Such strip is called an igniter. It burns away in a very short time when an arc forms producing the light, and lasts, if all goes well, until the candle burns down to its end. Without the igniter the current would not start and no arc would form.

I. H. P. Symbol for indicated horse-power.

Illuminating Power. The relative light given by any source compared with a standard light, and stated in terms of the same, as a burner giving an illuminating power of sixteen candles. For standards see Candle, Carcel—Methven Standard—Pentane Standard.

Illuminating Power, Spherical. The illuminating power of a lamp or source of light may vary in different directions, as in the case of a gas burner or incandescent lamp. The average illuminating power determined by photometric test or by calculation in all directions from the source of light is called the spherical illuminating power, or if stated in candles is called the spherical candle power.

Illumination, Unit of. An absolute standard of light received by a surface. Preece proposed as such the light received from a standard candle (see Candle, Standard) at a distance of 12.7 inches. The object of selecting this distance was to make it equal to the Carcel Standard (see Carcel), which is the light given by a Carcel lamp at a distance of one meter.

From one-tenth to one-fiftieth this degree of illumination was found in gas-lighted streets by Preece, depending on the proximity of the gas lamps.

Image, Electric. An electrified point or system of points on one side of a surface which would produce on the other side of that surface the same electrical action which the actual electrification of that surface really does produce. (Maxwell.)

The method of investigating the distribution of electricity by electric images is due to Sir William Thomson. The conception is purely a theoretical one, and is of mathematical value and interest.


Impedance. The ratio of any impressed electro-motive force to the current which it produces in a conductor. For steady currents it is only the resistance. For variable currents it may include besides resistance inductance and permittance. It is the sum of all factors opposing a current, both ohmic and spurious resistances. It is often determined and expressed as ohms.

Synonym—Apparent Resistance—Virtual Resistance.

Impedance, Oscillatory. The counter-electro-motive force offered to an oscillatory discharge, as that of a Leyden jar. It varies with the frequency of the discharge current.

Synonym—Impulsive Impedance.

Impressed Electro-motive Force. The electro-motive force expending itself in producing current induction in a neighboring circuit.

Impulse. (a) An electro-magnetic impulse is the impulse produced upon the luminiferous ether by an oscillatory discharge or other varying type of current; the impulse is supposed to be identical, except as regards wave-length, with a light wave.

(b) An electro-motive impulse is the electro-motive force which rises so high as to produce an impulsive or oscillatory discharge, such as that of a Leyden jar.

Incandescence, Electric. The heating or a conductor to red, or, more etymologically, to white heat by the passage of an electric current. The practical conditions are a high intensity of current and a low degree of conductance of the conductor relatively speaking.

Inclination Map. A map showing the locus of equal inclination or dips of the magnetic needle. The map shows a series of lines, each one of which follows the places at which the dip of the magnetic needle is identical. The map changes from year to year. (See Magnetic Elements.)

Independence of Currents in Parallel Circuits. If a number of parallel circuits of comparatively high resistance are supplied by a single generator of comparatively low resistance, the current passed through each one will be almost the same whether a single one or all are connected. Under the conditions named the currents are practically independent of each other.

[Transcriber's note: The current in each parallel branch depends on the resistance/impedance of that branch. Only if they all have the same impedance will the current be the same.]

Indicating Bell. An electric bell arranged to drop a shutter or disclose in some other way a designating number or character when rung.


Indicator. (a) An apparatus for indicating the condition of a distant element, such as the water level in a reservoir, the temperature of a drying room or cold storage room or any other datum. They are of the most varied constructions.

(b) The receiving instrument in a telegraph system is sometimes thus termed.

Indicator, Circuit. A galvanometer used to show when a circuit is active, and to give an approximate measurement of its strength. It is a less accurate and delicate form of instrument than the laboratory appliance.

Inductance. The property of a circuit in virtue of which it exercises induction and develops lines of force. It is defined variously. As clear and satisfactory a definition as any is the following, due to Sumpner and Fleming: Inductance is the ratio between the total induction through a circuit to the current producing it. "Thus taking a simple helix of five turns carrying a current of two units, and assuming that 1,000 lines of force passed through the central turn, of which owing to leakage only 900 thread the next adjacent on each side, and again only 800 through the end turns, there would be 800 + 900 + 1000 + 900 + 800, or 4,400 linkages of lines with the wire, and this being with 2 units of current, there would be 2,200 linkages with unit current, and consequently the self-inductance of the helix would be 2,200 centimetres." (Kennelly.) Inductance, as regards its dimensions is usually reduced to a length, hence the last word of the preceding quotation.

The practical unit of inductance is termed the henry, from Prof. Joseph Henry; the secohm, or the quad or quadrant. The latter alludes to the quadrant of the earth, the value in length of the unit in question.

[Transcriber's note: (L (di/dt) = V). A current changing at the rate of one ampere per second through a one henry inductance produces one volt. A sinusoidal current produces a voltage 90 degrees ahead of the current, a cosine (the derivative of sine is cosine). One volt across one henry causes the current to increase at one ampere per second.]

Induction, Coefficient of Self. The coefficient of self-induction of a circuit is the quantity of induction passing through it per unit current in it. If a given circuit is carrying a varying current it is producing a varying quantity of magnetic induction through itself. The quantity of induction through the circuit due to its current is generally proportional to its current. The quantity for unit current is the coefficient of self-induction. (Emtage.)

Induction, Cross. The induction of magnetic lines of force in a dynamo armature core by the current passing around such armature. These lines in a symmetrical two pole machine are at right angles to the lines of force which would normally extend across the space between the two magnet poles. The joint magnetizing effect of the field and of the cross induction produces a distorted field between the poles .

Synonym—Cross-magnetizing Effect.


Induction, Electro-magnetic. The inter-reaction of electromagnetic lines of force with the production of currents thereby.

A current passing through a conductor establishes around it a field of force representing a series of circular lines of force concentric with the axis of the conductor and perpendicular thereto. These lines of force have attributed to them, as a representative of their polarity, direction. This is of course purely conventional. If one is supposed to be looking at the end of a section of conductor, assuming a current be passing through it towards the observer, the lines of force will have a direction opposite to the motion of the hands of a watch. The idea of direction may be referred to a magnet. In it the lines of force are assumed to go from the north pole through the air or other surrounding dielectric to the south pole.

Two parallel wires having currents passing through them in the same direction will attract each other. This is because the oppositely directed segments of lines of force between the conductors destroy each other, and the resultant of the two circles is an approximation to an ellipse. As lines of force tend to be as short as possible the conductors tend to approach each other to make the ellipse become of as small area as possible, in other words to become a circle.

If on the other hand the currents in the conductors are in opposite directions the segments of the lines of force between them will have similar directions, will, as it were, crowd the intervening ether and the wires will be repelled.


By Ampre's theory of magnetism, (see Magnetism, Ampre's Theory of,) a magnet is assumed to be encircled by currents moving in the direction opposite to that of the hands of a watch as the observer faces the north pole. A magnet near a wire tends to place the Amprian currents parallel to the wire, and so that the portion of the Amprian currents nearest thereto will correspond in direction with the current in the wire.


This is the principle of the galvanometer. A number of methods of memoria technica have been proposed to remember it by.

Thus if we imagine a person swimming with the current and always facing the axis of the conductor, a magnetic needle held where the person is supposed to be will have its north pole deflected to the right hand of the person.


Again if we think of a corkscrew, which as it is turned screws itself along with the current, the motion of the handle shows the direction of the lines of force and the direction in which the north pole of a needle is deflected. This much is perhaps more properly electro-dynamics, but is necessary as a basis for the expression of induction.

If a current is varied in intensity in one conductor it will induce a temporary current in another conductor, part of which is parallel to the inducing current and which conductor is closed so as to form a circuit. If the inducing current is decreased the induced current in the near and parallel portion of the other circuit will be of identical direction; if increased the induced current will be of opposite direction.

This is easiest figured by thinking of the lines of force surrounding the inducing conductor. If the current is decreased these can be imagined as receiving a twist or turn contrary to their normal direction, as thereby establishing a turn or twist in the ether surrounding the other wire corresponding in direction with the direction of the original lines of force, or what is the same thing, opposite in direction to the original twist. But we may assume that the establishment of such a disturbance causes a current, which must be governed in direction with the requirements of the new lines of force.

The same reasoning applies to the opposite case.


The general statement of a variable current acting on a neighboring circuit also applies to the approach or recession of an unvarying current, and to the cutting of lines of force by a conductor at right angles thereto. For it is evident that the case of a varying current is the case of a varying number of lines of force cutting or being cut by the neighboring conductor. As lines of force always imply a current, they always imply a direction of such current. The cutting of any lines of force by a closed conductor always implies a change of position with reference to all portions of such conductor and to the current and consequently an induced current or currents in one or the other direction in the moving conductor.

As the inducing of a current represents energy abstracted from that of the inducing circuit, the direction of the induced current is determined by (Lenz's Law) the rule that the new current will increase already existing resistances or develop new ones to the disturbance of the inducing field.

In saying that a conductor cutting lines of force at right angles to itself has a current induced in it, it must be understood that if not at right angles the right angle component of the direction of the wire acts in generating the current. The case resolves itself into the number of lines of force cut at any angle by the moving wire.

The lines of force may be produced by a magnet, permanent or electro. This introduces no new element. The magnet may be referred, as regards direction of its lines of force, to its encircling currents, actual or Amprian, and the application of the laws just cited will cover all cases.

Induction, Coefficient of Mutual. The coefficient of mutual induction of two circuits is the quantity of magnetic induction passing through either of them per unit current in the other. (Emtage.) It is also defined as the work which must be done on either circuit, against the action of unit current in each, to take it away from its given position to an infinite distance from the other; and also as the work which would be done by either circuit on the other in consequence of unit current in each, as the other moves from an infinite distance to its given position with respect to the other conductor. It depends on the form, size, and relative position of the two circuits; and on the magnetic susceptibilities of neighboring substances.

The ether surrounding two circuits of intensity i' and i" must possess energy, expressible (Maxwell) as 1/2 L i2 + M i i + 1/2 N i12. It can be shown that M i i1 in any given position of the two circuits is numerically equal (1) to the mutual potential energy of the two circuits (2) to the number of lines of induction, which being due to A, pass from A through B, or equally being due to B, pass from B through A, and M is styled the coefficient of mutual induction. (Daniell.)


Induction, Electrostatic. An electrostatic charge has always an opposite and bound charge. This may be so distributed as not to be distinguishable, in which case the charge is termed, incorrectly but conventionally, a free charge. But when a charge is produced an opposite and equal one always is formed, which is the bound charge. The region between the two charges and permeated by their lines of force, often curving out so as to embrace a volume of cross-sectional area larger than the mean facing area of the excited surfaces, is an electrostatic field of force. The establishing of an electrostatic field, and the production of a bound charge are electrostatic induction.

An insulated conductor brought into such a field suffers a redistribution of its electricity, or undergoes electrostatic induction. The parts nearest respectively, the two loci of the original and the bound charges, are excited oppositely to such charges. The conductor presents two new bound charges, one referred to the original charge, the other to the first bound charge.

Induction, Horizontal. In an iron or steel ship the induction exercised upon the compass needle by the horizontal members of the structure, such as deck-beams, when they are polarized by the earth's magnetic induction. This induction disappears four times in swinging a ship through a circle; deviation due to it is termed quadrantal deviation. (See Deviation, Quadrantal.)

Induction, Lateral. A term formerly used to express the phenomenon of the alternative discharge of a Leyden jar or other oscillatory discharge of electricity. (See Discharge, Alternative.)

Induction, Magnetic. The magnetization of iron or other paramagnetic substance by a magnetic field.

On account of its permeability or multiplying power for lines of force, a paramagnetic body always concentrates lines of force in itself if placed in a magnetic field, and hence becomes for the time being a magnet, or is said to be polarized.

As the tendency of lines of force is to follow the most permeable path, a paramagnetic bar places itself lengthwise or parallel with the prevailing direction of the lines of force so as to carry them as far on their way as possible. Every other position of the bar is one of unstable equilibrium or of no equilibrium. The end of the bar where the lines of force enter (see Lines of Force) is a south pole and is attracted towards the north pole of the magnet.

The production of magnetic poles under these conditions in the bar is shown by throwing iron filings upon it. They adhere to both ends but not to the middle.

Induction, Mutual, Electro-magnetic. The induction due to two electric currents reacting on each other.


Induction, Mutual, Electrostatic. A charged body always induces a charge upon any other body near it; and the same charge in the second body will induce the other charge in the first body if the latter is unexcited. In other words the second body's induction from the first is the measure of the charge the second would require to induce in the first its own (the second's) induced charge. This is the law of mutual electrostatic induction.

Induction, Open Circuit. Inductive effects produced in open circuits. By oscillatory discharges a discharge can be produced across a break in a circuit otherwise complete. The requirements for its production involve a correspondence or relation of its dimensions to the inducing discharge. The whole is analogous to the phenomena of sound resonators and sympathetic vibrations. Synonym—Oscillatory induction.

Induction, Self-. (a) A phenomenon of electric currents analogous to the inertia of matter. Just as water which fills a pipe would resist a sudden change in its rate of motion, whether to start from rest, to cease or decrease its motion, so an electric current requires an appreciable time to start and stop. It is produced most strongly in a coiled conductor, especially if a core of iron is contained within it.

As in the case of two parallel wires, one bearing currents which vary, momentary currents are induced in the other wire, so in a single conductor a species of inertia is found which retards and prolongs the current. If a single conductor is twisted into a helix or corresponding shape, its separate turns react one on the other in accordance with the general principles of electromagnetic induction. (See Induction, Electro-magnetic.) Thus when a current is suddenly formed the coils acting upon each other retard for an instant its passage, producing the effect of a reverse induced current or extra current opposing the principal current. Of course no extra current is perceptible, but only the diminution. When the current is passing regularly and the current is broken, the corresponding action prolongs the current or rather intensifies it for an instant, producing the true extra current. This is current self-induction.

[Transcriber's note: See inductance.]

Synonyms—Electric Inertia—Electro-dynamic Capacity.

(b) A permanent magnet is said to tend to repel its own magnetism, and thus to weaken itself; the tendency is due to magnetic self-induction.

Induction Sheath. In the brush dynamo a thin sheet of copper surrounding the magnet cores with edges soldered together. The winding is outside of it. Its object is to absorb extra currents set up by variations in magnetic intensity in the cores. These currents otherwise would circulate in the cores.


Induction, Unit of Self-. The unit of self-induction is the same as that of induction in general. It is the henry, q. v.

Induction, Unipolar. Induction produced in a conductor which continuously cuts the lines of force issuing from one pole of a magnet. As the lines of force are always cut in the same sense a continuous and constant direction current is produced.

Induction, Vertical. In an iron or steel ship the induction or attraction exercised in the compass by vertical elements of the structure. Such vertical masses of iron in the northern hemisphere would have their upper ends polarized as south poles, and would affect the magnet as soon as the vessel swung out of the magnetic meridian. Thus this induction disappears twice in swinging a ship through a complete circle; deviation due to it is termed semi-circular deviation. (See Deviation, Semi-circular.)


Inductophone. A method of train telegraphy. The train carries a circuit including a coil, and messages are picked up by it from coils along the line into which an alternating current is passed. A telephone is used as a receiver in place of a sounder or relay. The invention, never practically used, is due to Willoughby Smith.


Inductor. (a) In a current generator a mass of iron, generally laminated, which is moved past a magnet pole to increase the number of lines of force issuing therefrom. It is used in inductor dynamos. (See Dynamo Inductor.) In the cut Fig. 202, of an inductor dynamo i, i, are the laminated inductors.

(b) In influence machines the paper or tinfoil armatures on which the electrification is induced.

Inertia. A force in virtue of which every body persists in its state of motion or rest except so far as it is acted on by some force.

Inertia, Electro-magnetic. This term is sometimes applied to the phenomena of self-induction, or rather to the cause of these phenomena.

Infinity Plug. A plug in a resistance box, which on being pulled out of its seat opens the circuit or makes it of infinite resistance. The plug seats itself between two brass plates which are not connected with each other in any way. The other plates are connected by resistance coils of varying resistance.

Influence, Electric. Electric induction, which may be either electrostatic, current, or electro-magnetic.

Insolation, Electric. Exposure to powerful arc-light produces effects resembling those of sun-stroke. The above term or the term "electric sun-stroke" has been applied to them.

[Transcriber's note: Operators of arc welders are prone to skin cancer from ultra violet rays if not properly protected.]

Installation. The entire apparatus, buildings and appurtenances of a technical or manufacturing establishment. An electric light installation, for instance, would include the generating plant, any special buildings, the mains and lamps.

Insulating Stool. A support for a person, used in experiments with static generators. It has ordinarily a wooden top and glass legs. It separates one standing on it from the earth and enables his surface to receive an electrostatic charge. This tends to make his hair stand on end, and anyone on the floor who touches him will receive a shock.

Insulating Tape. Prepared tape used in covering the ends of wire where stripped for making joints. After the stripped ends of two pieces are twisted together, and if necessary soldered and carefully cleaned of soldering fluid, they may be insulated by being wound with insulating tape.

The tape is variously prepared. It may be common cotton or other tape saturated with any insulating compound, or may be a strip of gutta percha or of some flexible cement-like composition.


Insulating Varnish. Varnish used to coat the surface of glass electrical apparatus, to prevent the deposition of hygrometric moisture, and also in the construction of magnetizing and induction coils and the like. Shellac dissolved in alcohol is much used. Gum copal dissolved in ether is another. A solution of sealing wax in alcohol is also used. If applied in quantities these may need baking to bring about the last drying. (See Shellac Varnish.)

Insulator. (a) Any insulating substance.

(b) A telegraph or line insulator for telegraph wires. (See Insulator, Line or Telegraph.)


Insulator Cap. A covering or hood, generally of iron, placed over an insulator to protect it from injury by fracture with stones or missiles.

Insulator, Fluid. (a) For very high potentials, as in induction coils or alternating circuits, fluid insulators, such as petroleum or resin oil, have been used. Their principal merit is that if a discharge does take place through them the opening at once closes, so that they are self-healing.

(b) Also a form of telegraph or line insulator in which the lower rim is turned up and inwards, so as to form an annular cup which is filled with oil.

Insulator, Line or Telegraph. A support often in the shape of a collar or cap, for a telegraph or other wire, made of insulating material. Glass is generally used in the United States, porcelain is adopted for special cases; pottery or stone ware insulators have been used a great deal in other countries. Sometimes the insulator is an iron hook set into a glass screw, which is inserted into a hole in a telegraph bracket. Sometimes a hook is caused to depend from the interior of an inverted cup and the space between the shank of the hook and cup is filled with paraffine run in while melted.

Insulators are tested by measuring their resistance while immersed in a vessel of water.

Intensity. Strength. The intensity of a current or its amperage or strength; the intensity or strength of a magnetic field or its magnetic density; the intensity or strength of a light are examples of its use. In the case of dynamic electricity it must be distinguished from tension. The latter corresponds to potential difference or voltage and is not an attribute of current; intensity has no reference to potential and is a characteristic of current.

Intensity of a Magnetic Field. The intensity of a magnetic field at any point is measured by the force with which it acts on a unit magnet pole placed at that point. Hence unit intensity of field is that intensity of field which acts on a unit pole with a force of one dyne. (S. P. Thomson.) (See Magnetic Lines of Force.)


Intercrossing. Crossing a pair of conductors of a metallic circuit from side to side to avoid induction from outside sources.

Intermittent. Acting at intervals, as an intermittent contact, earth, or grounding of a telegraph wire.

Interpolar Conductor. A conductor connecting the two poles of a battery or current generator; the external circuit in a galvanic circuit.

Interpolation. A process used in getting a closer approximation to the truth from two varying observations, as of a galvanometer. The process varies for different cases, but amounts to determining an average or deducing a proportional reading from the discrepant observed ones.

Interrupter. A circuit breaker. It may be operated by hand or be automatic. (See Circuit Breaker—Circuit Breaker, Automatic—and others.)

Interrupter, Electro-magnetic, for a Tuning Fork. An apparatus for interrupting a current which passes through an electromagnet near and facing one of the limbs of a tuning fork. The circuit is made and broken by the vibrations of another tuning fork through which the current passes. The second one is thus made to vibrate, although it may be very far off and may not be in exact unison with the first. The first tuning fork has a contact point on one of its limbs, to close the circuit; it may be one which dips into a mercury cup.

Intrapolar Region. A term in medical electricity, denoting the part of a nerve through which a current is passing.

Ions. The products of decomposition produced in any given electrolysis are termed ions, the one which appears at the anode or negative electrode is the anion. The electrode connected to the carbon or copper plate of a wet battery is an anode. Thus in the electrolysis of water oxygen is the anion and hydrogen is termed the kation. In this case both anion and kation are elements. In the decomposition of copper sulphate the anion is properly speaking sulphion (S O4), a radical, and the kation is copper, an element. Electro-negative elements or radicals are anions, such as oxygen, sulphion, etc., while electro-positive ones are kations, such as potassium. Again one substance may be an anion referred to one below it and a kation referred to one above it, in the electro-chemical series, q. v. Anion means the ion which goes to the anode or positive electrode; kation, the ion which goes to the kathode or negative electrode.

[Transcriber's note: An ion is an atom or molecule that has lost or gained one or more valence electrons, giving it a positive or negative electrical charge. A negatively charged ion, with more electrons than protons in its nuclei, is an anion. A positively charged ion, with fewer electrons than protons, is a cation. The electron was discovered five years after this publication.]


Iron. A metal; one of the elements; symbol, Fe; atomic weight, 56; equivalent, 28 and 14, ; valency, 4 and 2. It is a conductor of electricity. The following data are at 0 C. 32 F., with annealed metal.

Specific Resistance, 9.716 microhms. Relative Resistance. 6.460 Resistance of a wire, (a) 1 foot long weighing 1 grain, 1.085 ohms. (b) 1 foot long 1/1000 inch thick, 58.45 " (c) 1 meter long weighing 1 gram, .7570 " (d) 1 meter long, 1 millimeter thick, .1237 " Percentage increase in resistance per degree C. (1.8 F.) at about 20 C. (68F.), about 0.5 per cent. Resistance of a 1 inch cube, 3.825 microhms. Electro-chemical equivalent (Hydrogen = .0105), .147 and .294

Iron, Electrolytic. Iron deposited by electrolytic action. Various baths are employed for its formation. (See Steeling.) It has very low coercive power, only seven to ten times that of nickel.

Ironwork Fault of a Dynamo. A short circuiting of a dynamo by, or any connection of its coils with, the iron magnet cores or other iron parts.

Isochronism. Equality of periodic time; as of the times of successive beats of a tuning fork, or of the times of oscillations of a pendulum.

Isoclinic Lines. The lines denoting the locus of sets of equal dips or inclinations of the magnetic needle upon the earth's surface, the magnetic parallels, q. v. These lines are very irregular. (See Magnetic Elements.)

Isoclinic Map. A map showing the position of isoclinic lines.

Isodynamic Lines. Lines marking the locus of places of equal magnetic intensity on the earth's surface. (See Magnetic Elements, Poles of Intensity.)

Isodynamic Map. A map showing the position of isodynamic lines. (See Poles of Intensity.)

Isogonic Lines. Lines on a map marking the locus of or connecting those points where the declination or variation of the magnetic needle is the same. (See Magnetic Elements—Declination of Magnetic Needle.)

Synonyms—Isogonal Lines—Halleyan Lines.


Isogonic Map. A map showing the isogonic lines. On such a map each line is characterized and marked with the degrees and direction of variation of the compass upon itself.

Synonym—Declination Map.

Isolated Plant, Distribution or Supply. The system of supplying electric energy by independent generating systems, dynamo or battery, for each house, factory or other place, as contra-distinguished from Central Station Distribution or Supply.

Isotropic. (Greek, equal in manner.)

Having equal properties in all directions; the reverse of anisotropic, q. v. Thus a homogeneous mass of copper or silver has the same specific resistance in all directions and is an isotropic conductor. Glass has the same specific inductive capacity in all directions and is an isotropic medium or dielectric. The same applies to magnetism. Iron is an isotropic paramagnetic substance. (See Anisotropic.) The term applies to other branches of physics also.

I. W. G. Contraction for Indian Wire Gauge—the gauge adopted in British India.

J. Symbol for the unit joule, the unit of electric energy.

Jacobi's Law. A law of electric motors. It states that the maximum work of a motor is performed when the counter-electromotive force is equal to one-half the electro-motive force expended on the motor.

Jewelry. Small incandescent lamps are sometimes mounted as articles of jewelry in scarf-pins or in the hair. They may be supplied with current from storage or from portable batteries carried on the person.

Joint, American Twist. A joint for connecting telegraph wires, especially aerial lines. Its construction is shown in the cut. The end of each wire is closely wound around the straight portion of the other wire for a few turns.



Joint, Britannia. A joint for uniting the ends of telegraph and electric wires. The ends of the wires are scraped clean and laid alongside each other for two inches, the extreme ends being bent up at about right angles to the wire. A thin wire is wound four or five times around one of the wires, back of the joint, the winding is then continued over the lapped portion, and a few more turns are taken around the other single wire. The whole is then soldered.


Joint, Butt. A joint in belting or in wire in which the ends to be joined are cut off square across, placed in contact and secured. It ensures even running when used in belting. Any irregularity in thickness of a belt affects the speed of the driven pulley. As dynamos are generally driven by belts, and it is important to drive them at an even speed to prevent variations in the electro-motive force, butt joints should be used on belting for them, unless a very perfect lap joint is made, which does not affect either the thickness or the stiffness of the belt.

When a butt joint is used in wire a sleeve may be used to receive the abutting ends, which may be secured therein by soldering. This species of joint has been used on lightning rods and may more properly be termed a sleeve joint.

Joint, Lap. (a) In belting a joint in which the ends are overlapped, and riveted or otherwise secured in place. If made without reducing the thickness of the ends it is a bad joint for electrical work, as it prevents even running of machinery to which it is applied. Hence dynamo belts should be joined by butt joints, or if by lap joints the ends should be shaved off so that when joined and riveted, there will be no variation in the thickness of the belt.

(b) In wire lap joints are made by overlapping the ends of the wire and soldering or otherwise securing. The Britannia joint (see Joint, Britannia,) may be considered a lap-joint.

Joint, Marriage. A joint for stranded conductors used for Galende's cables. It is made somewhat like a sailor's long splice. Each one of the strands is wound separately into the place whence the opposite strand is unwound and the ends are cut off so as to abutt. In this way all are smoothly laid in place and soldering is next applied.



Joint, Sleeve. A joint in electric conductors, in which the ends of the wires are inserted into and secured in a metallic sleeve or tube, whose internal diameter is just sufficient to admit them.

Joint, Splayed. The method of joining the ends of stranded conductors. The insulating covering is removed, the wires are opened out, and the center wire, heart or core of the cable is cut off short. The two ends are brought together, the opened out wires are interlaced or crotched like the fingers of the two hands, and the ends are wound around the body of the cable in opposite directions. The joint is trimmed and well soldered. Tinned wire with rosin flux for the soldering is to be recommended. Insulating material is finally applied by hand, with heat if necessary.

Joints in Belts. Belt-joints for electric plants where the belts drive dynamos should be made with special care. The least inequality affects the electro-motive force. Butt joints are, generally speaking, the best, where the ends of the belt are placed in contact and laced. Lap-joints are made by overlapping the belt, and unless the belt is carefully tapered so as to preserve uniform strength, the speed of the dynamo will vary and also the electromotive force.

Joulad. A name proposed to be substituted for "joule," q. v. It has not been adopted.

Joule. This term has been applied to several units.

(a) The practical C. G. S. unit of electric energy and work—the volt-coulomb. It is equal to 1E7 ergs—0.73734 foot pound.—.00134 horse power seconds. A volt-ampere represents one joule per second.

(b) It has also been used as the name of the gram-degree C. thermal unit—the small calorie.


Joule Effect. The heating effect of a current passing through a conductor. It varies with the product of the resistance by the square of the current, or with (C^2)*R.

Joule's Equivalent. The mechanical equivalent of heat, which if stated in foot-pounds per pound-degree F. units, is 772 (772.55). (See Equivalents.)

Junction Box. In underground distribution systems, an iron casing or box in which the feeders and mains are joined, and where other junctions are made.

Synonym—Fishing Box.

K. The symbol for electrostatic capacity.

Kaolin. A product of decomposition of feldspar, consisting approximately of silica, 45, alumina, 40, water, 15. It was used in electric candles of the Jablochkoff type as a constituent of the insulating layer or colombin. Later it was abandoned for another substance, as it was found that it melted and acted as a conductor.


Kapp Line of Force. A line of force proposed by Kapp. It is equal to 6,000 C. G. S. lines of force, and the unit of area is the square inch. Unfortunately it has been adopted by many manufacturers, but its use should be discouraged, as it is a departure from the uniform system of units.

One Kapp line per square inch = 930 C. G. S. lines per square centimeter.

Kathelectrotonus. A term used in medical electricity or electro-therapeutics to indicate the increased functional activity induced in a nerve by the proximity of the kathode of an active circuit which is completed through the nerve. The converse of anelectrotonus.

Kathode. The terminal of an electric circuit whence an electrolyzing current passes from a solution. It is the terminal connected to the zinc plate of a primary battery.

Kathodic Closure Contraction. A term in electro-therapeutics; the contractions near where the kathode of an active circuit is applied to the body, which are observed at the instant when the circuit is closed.

Kathodic Duration Contraction. A term in electro-therapeutics; the contraction near where the kathode of an active circuit is applied to the body for a period of time.

K. C. C. Abbreviation for Kathodic Closure Contraction, q. v.

K. D. C. Abbreviation for Kathodic Duration Contraction, q. v.

Keeper. A bar of soft iron used to connect the opposite poles of a horseshoe magnet or the opposite poles of two bar magnets placed side by side. It is designed to prevent loss of magnetism. The armature of a horseshoe magnet is generally used as its keeper. For bar magnets a keeper is used for each end, the magnets being laid side by side, with their poles in opposite direction but not touching, and a keeper laid across at each end connecting the opposite poles.

Kerr Effect. The effect of an electrostatic field upon polarized light traversing a dielectric contained within the field. (See Electrostatic Refraction.)

Kerr's Experiment. Polarized light reflected from the polished face of a magnet pole has its plane of polarization rotated; when it is reflected from the north pole the rotation is from left to right.


Key. A switch adapted for making and breaking contact easily when worked by hand, as a Morse telegraph key.

Key Board. A board or tablet on which keys or switches are mounted.

Key-board. (a) A switch board, q. v.

(b) A set of lettered keys similar to those of a typewriter employed in some telegraph instruments. As each key is depressed it produces the contact or break requisite for the sending of the signal corresponding to the letter marked upon the key. The signal in printing telegraphs, on which such key-boards are used, is the reprinting of the letter at the distant end of the line.

Key, Bridge. A key for use with a Wheatstone Bridge, q.v. It is desirable to first send a current through the four arms of the bridge in using it for testing resistances and then through the galvanometer, because it takes a definite time for the current to reach its full strength. This is especially the case if the element being measured has high static capacity, as a long ocean cable. If the galvanometer connections were completed simultaneously with the bridge connections a momentary swing would be produced even if the arms bore the proper relation to each other. This would cause delay in the testing. A bridge key avoids this by first connecting the battery circuit through the arms of the bridge, and then as it is still further depressed the galvanometer circuit is completed.



Key, Charge and Discharge. A key for use in observing the discharge of a condenser immediately after removing the battery. In one typical form it has two contacts, one below and one above, and being a spring in itself is pressed up against the upper one. Connections are so made that when in its upper position it brings the two coatings of the condenser in circuit with the galvanometer. When depressed it does the same for a battery. In use it is depressed and suddenly released when the galvanometer receives the full charge, before there has been time for leakage. This is one method of connection illustrating its principle.

In the cut L is the spring-key proper. S2, is the upper contact screw against which the spring normally presses. In this position the galvanometer G is in circuit with the opposite coatings of the condenser C. On depressing the contact S2, is broken and S1, is made. This brings the battery B in circuit with the condenser coatings. On releasing the key it springs up and the galvanometer receives the effect of the charge of the condenser as derived from the battery.

Key, Double Contact. A key arranged to close two distinct circuits, holding the first closed until the second is completed. It is used for Wheatstone bridge work.

Key, Double Tapper. A telegraph key giving contacts alternately for currents in opposite directions, used in needle telegraphy.

Key, Increment. A key for use in duplex and quadruplex telegraphy. Its action is to increase the line current, not merely to suddenly turn current into it.



Key, Kempe's Discharge. A key giving a charging, discharging and insulating connection, for static condenser work. Referring to the cut l is a lever or spring with upper discharging contact s, and lower charging contact s'. In use it is pressed down by the insulating handle or finger piece C, until caught by the hook attached to the key I. This hook is lower down than that on the key D, and holds it in contact with the charging contact piece S'. On pressing the key I, marked or designated "Insulate," it springs up, breaks contact at S', and catching against the hook on D, which key is designated "Discharge," remains insulated from both contacts; next on pressing D it is released and springs up and closes the discharge contact S. It is a form of charge and discharge key. (See Key, Charge and Discharge.)

Key, Magneto-electric. A telegraph key whose movements operate what is virtually a small magneto-generator, so as to produce currents of alternating direction, one impulse for each motion of the key. It is employed for telegraphing without a line battery, a polarized relay being used. In one very simple form a key is mounted on a base with a permanent magnet and connected to the armature, so that when the key is pressed downwards it draws the armature away from the poles of the magnet. If the magnet or its armature is wound with insulated wire this action of the key will cause instantaneous currents to go through a circuit connected to the magnet or armature coils.


In Siemens & Halske's key an H armature E is pivoted between the poles N S, of a powerful compound horseshoe magnet, G G. It is wound with fine wire and a key handle H is provided for working it. In its normal position the handle is drawn upward, and the end S S of the armature core is in contact with the south pole S of the permanent magnet, and the end D D with the north pole. This establishes the polarity of the armature. On depressing the key the contacts are broken and in their place the end D D comes in contact with the south pole and the end S S with the north pole. This suddenly reverses the polarity of the armature and sends a momentary current through the armature coil which is in circuit with the line. The cut only shows the principle of the key, whose construction is quite complicated.


Key, Make and Break. An ordinary electric key, usually making a contact when depressed, and rising by spring action when released, and in its rise breaking the contact.

Fig. 209. PLUG KEY

Key, Plug. An appliance for closing a circuit. Two brass blocks are connected to the terminals, but are disconnected from each other. A brass plug slightly coned or with its end split so as to give it spring action is thrust between the blocks to complete the circuit. It is used in Resistance coils and elsewhere. (See Coil, Resistance.) Grooves are formed in the blocks to receive the plug.

Key, Reversing. (a) A double key, arranged so that by depressing one key a current flows in one direction, and by depressing the other a current flows in the opposite direction. It is used in connection with a galvanometer in experimental, testing or measuring operations.

(b) A key effecting the same result used in quadruplex telegraphy.

Key, Sliding-Contact. A name given to the key used for making instantaneous contacts with the metre wire of a metre bridge, q. v. The name is not strictly correct, because it is important that there should be no sliding contact made, as it would wear out the wire and make it of uneven resistance.

It is a key which slides along over the wire and which, when depressed, presses a platinum tipped knife edge upon the wire. On being released from pressure the key handle springs up and takes the knife edge off the wire. This removal is essential to avoid wearing the wire, whose resistance per unit of length must be absolutely uniform.

Key, Telegraph. The key used in telegraphy for sending currents as desired over the line. It consists of a pivoted lever with finger piece, which lever when depressed makes contact between a contact point on its end and a stationary contact point on the base. This closes the circuit through the line. When released it springs up and opens the line circuit.

Kilo. A prefix to the names of units; it indicates one thousand times, as kilogram, one thousand grams. A few such units are given below.

Kilodyne. A compound unit; one thousand dynes. (See Dyne.)

Kilogram. A compound unit; one thousand grams; 2.2046 pounds avds.


Kilojoule. A compound unit; one thousand joules, q. v.

Kilometer. A compound unit; one thousand meters; 3280.899 feet; 0.621382 statute miles. (See Meter.)

Kilowatt. A compound unit; one thousand watts, q. v.

Kine. An absolute or C. G. S. unit of velocity or rate of motion; one centimeter per second; proposed by the British Association.

Kirchoff's Laws. These relate to divided circuits.

I. When a steady current branches, the quantity of electricity arriving by the single wire is equal to the quantity leaving the junction by the branches. The algebraical sum of the intensities of the currents passing towards (or passing from) the junction is equal to zero; Summation(C) = 0 (Daniell.) In the last sentence currents flowing towards the point are considered of one sign and those flowing away from it of the other.

II. In a metallic circuit comprising within it a source of permanent difference of potential, E, the products of the intensity of the current within each part of the circuit into the corresponding resistance are, if the elements of current be all taken in cyclical order together, equal to E; Summation(C * r) =E. In a metallic circuit in which there is no source of permanent difference of potential E = 0, and Summation(C * r) = 0.

This law applies to each several mesh of a wire network as well as to a single metallic loop, and it holds good even when an extraneous current is passed through the loop. (Daniell.)

In this statement of the two laws E stands for electro-motive force, C for current intensity; and r for resistance of a single member of the circuit.

[Transcriber's note: These laws may be restated as: At any point in an steady-state electrical circuit, the directed sum of currents flowing towards that point is zero. The directed sum of the electrical potential differences around any closed circuit is zero.]

Knife-edge Suspension. The suspension of an object on a sharp edge of steel or agate. The knife edge should abut against a plane. The knife edge is generally carried by the poised object. Its edge then faces downward and on the support one or more plane or approximately plane surfaces are provided on which it rests. In the ordinary balance this suspension can be seen. It is sometimes used in the dipping needle.

It is applied in cases where vertical oscillations are to be provided for.

Knot. The geographical mile; a term derived from the knots on the log line, used by navigators. It is equal to 6,087 feet.

Synonyms—Nautical Mile—Geographical Mile.

[Transcriber's note: A knot is a velocity, 1 nautical mile per hour, not a distance. The contemporary definition is: 1 international knot = 1 nautical mile per hour = 1.852 kilometres per hour = 1.1507794 miles per hour = 0.51444444 meters per second = 6076.1152 feet per hour.]


Kohlrausch's Law. A law of the rate of travel of the elements and radicals in solutions under the effects of electrolysis. It states that each element under the effects of electrolysis has a rate of travel for a given liquid, which is independent of the element with which it was combined. The rates of travel are stated for different elements in centimeters per hour for a potential difference of one or more volts per centimeter of path.

[Friedrich Wilhelm Georg Kohlrausch (1840-1910)]

Kookogey's Solution. An acid exciting and depolarizing solution for a zinc-carbon couple, such as a Bunsen battery. Its formula is: Potassium bichromate, 227 parts; water, boiling, 1,134 parts; while boiling add very carefully and slowly 1,558 parts concentrated sulphuric acid. All parts are by weight. Use cold.

Krizik's Cores. Cores of iron for use with magnetizing coils, q. v. They are so shaped, the metal increasing in quantity per unit of length, as the centre is approached, that the pull of the excited coil upon them will as far as possible be equal in all positions. A uniform cylinder is attracted with varying force according to its position; the Krizik bars or cores are attracted approximately uniformly through a considerable range.

L. Symbol for length and also for the unit of inductance or coefficient of induction, because the dimensions of inductance are length.

Lag, Angle of. (a) The angle of displacement of the magnetic axis of an armature of a dynamo, due to its magnetic lag. The axis of magnetism is displaced in the direction of rotation. (See Magnetic Lag.)

(b) The angle expressing the lag of alternating current and electro-motive force phases.

Laminated. adj. Made up of thin plates, as a laminated armature core or converter core.

Lamination. The building up of an armature core or other thing out of plates. The cores of dynamo armatures or of alternating current converters are often laminated. Thus a drum armature core may consist of a quantity of thin iron discs, strung upon a rod and rigidly secured, either with or without paper insulation between the discs. If no paper is used the film of oxide on the iron is relied on for insulation. The object of lamination is to break up the electrical continuity of the core, so as to avoid Foucault currents. (See Currents, Foucault.) The laminations should be at right angles to the direction of the Foucault currents which would be produced, or in most cases should be at right angles to the active parts of the wire windings.


Lamination of Armature Conductors. These are sometimes laminated to prevent the formation of eddy currents. The lamination should be radial, and the strips composing it should be insulated from each other by superficial oxidation, oiling or enamelling, and should be united only at their ends.


Lamp, Arc. A lamp in which the light is produced by a voltaic arc. Carbon electrodes are almost universally employed. Special mechanism, operating partly by spring or gravity and partly by electricity, is employed to regulate the distance apart of the carbons, to let them touch when no current passes, and to separate them when current is first turned on.

The most varied constructions have been employed, examples of which will be found in their places. Lamps may in general be divided into classes as follows, according to their regulating mechanism and other features:

(a) Single light regulators or monophotes. Lamps through whose regulating mechanism the whole current passes. These are only adapted to work singly; if several are placed in series on the same circuit, the action of one regulator interferes with that of the next one.

(b) Multiple light regulators or polyphotes. In these the regulating mechanism and the carbons with their arc are in parallel; the regulating device may be a single magnet or solenoid constituting a derived or shunt-circuit lamp, or it may include two magnets working differentially against or in opposition to each other constituting a differential lamp.


(c) Lamps with fixed parallel carbons termed candles (q. v., of various types).

(d) Lamps without regulating mechanism. These include lamps with converging carbons, whose object was to dispense with the regulating mechanism, but which in some cases have about as much regulating mechanism as any of the ordinary arc lamps.

Lamp, Contact. A lamp depending for its action on loose contact between two carbon electrodes. At the contact a species of incandescence with incipient arcs is produced. One of the electrodes is usually flat or nearly so, and the other one of pencil shape rests upon it.

Lamp, Differential Arc. An arc lamp, the regulation of the distance between whose carbons depends on the differential action of two separate electrical coils. The diagram illustrates the principle. The two carbons are seen in black; the upper one is movable, The current arrives at A. It divides, and the greater part goes through the low resistance coil M to a contact roller r, and thence by the frame to the upper carbon, and through the arc and lower carbon to B, where it leaves the lamp. A smaller portion of the current goes through the coil M1 of higher resistance and leaves the lamp also at B. A double conical iron core is seen, to which the upper carbon holder is attached. This is attracted in opposite directions by the two coils. If the arc grows too long its resistance increases and the coil M1 receiving more current draws it down and thus shortens the arc. If the arc grows too short, its resistance falls, and the coil M receives more current and draws the core upwards, thus lengthening the arc. This differential action of the two cores gives the lamp its name. R is a pulley over which a cord passes, one end attached to the core and the other to a counterpoise weight, W.

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