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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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A phosphorescent body, after exposure to light, is luminous itself. Phosphorescence may be induced by rubbing or friction, by heat, by molecular bombardment, as in Crookes' tubes, and by static discharge of electricity, as well as by simple exposure to light.

Another form of phosphorescence may be due to slow chemical combustion. This is the cause of the luminosity of phosphorous.

Phosphorous, Electrical Reduction of. Phosphorous is reduced from bone phosphate by the heat of the electric arc. The phosphate mixed with charcoal is exposed to the heat of the voltaic are, and reduction of the phosphorous with its volatilization at once ensues. The phosphorous as it volatilizes is condensed and collected.

Photo-electricity. The development of electrical properties by exposure to light. Crystals of fluor spar are electrified not only by heat (see Pyro-electricity) but also by exposure to sunlight or to the light of the voltaic arc.

[Transcribers note: Although first observed in 1839 by Becquerel, it was not explained until 1905 by Albert Einstein with the introduction of photons.]

Photo-electric Microscope. A projection, solar or magic-lantern microscope worked by the electric light.

Photo-electro-motive Force. Electro-motive force produced in a substance by the action of light.

411 STANDARD ELECTRICAL DICTIONARY.

Photometer. An apparatus for measuring the intensity of light emitted by a given lamp or other source of illuminating power. They may be classified into several types.

Calorimetric or Heat Photometers act by measuring relatively the heat produced by the ether waves (so-called radiant heat) emitted by the source. The accuracy of the instrument is increased by passing the rays through an alum solution. A thermopile, or an air thermometer, may be used to receive the rays.

Chemical Photometers. In these the light falls upon sensitized photographic paper. The depth of coloration is used as the index of illuminating power.

Direct Visual Photometers. These include Rumford's Shadow Photometer, Bunsen's Bar Photometer, and Wheatstone's Bead Photometer, in which the light is estimated by direct visual comparison of its effects.

Optical Photometers. These include Polarization Photometers, in which the light is polarized; Dispersion Photometers, in which a diverging lens is placed in the path of the rays of light so as to reduce the illuminating power in more rapid ratio than that of the square of the distance.

Selenium Photometers, in which the variations in resistance of selenium as light of varying intensity falls upon it is used as the indicator of the intensity of the light.

Jet Photometers, for gas only, in which the height of a flame under given conditions, or the conditions requisite to maintain a flame of given height, is used to indicate the illuminating power.

The subject of photometers has acquired more importance than ever in view of the extensive introduction of the electric light. (See Candle, Standard—Carcel—Viol's Standard—and Photometers of various kinds.)

Photometer, Actinic. A photometer whose registrations are produced by the action of the light being tested upon sensitized paper or plates, such as used in photography. Some efforts at self-registering photometers have been based on actinic registration of the height of a flame of the gas to be tested.

Photometer, Bar. A photometer in which the two lights to be compared are fixed at or opposite to the ends of a bar or scale of known length, generally 60 or 100 inches. The bar is divided by the rule of the inverse square of the distances, so that if a screen is placed on any part of the bar where it receives an equal amount of light from both sources, the figure on the bar will indicate the relative illuminating power of the larger lamp or light in terms of the smaller. The divisions of the bar are laid out on the principle that the illuminating power of the two sources of light will vary inversely with the square of their distance from the screen.

412 STANDARD ELECTRICAL DICTIONARY.

The screen used is sometimes the Bunsen disc. This is a disc of paper with a spot of paraffine wax in the centre melted thoroughly into the paper or with a ring of paraffine wax surrounding the untouched centre. When this disc is equally illuminated on both sides the spot is nearly invisible. Inequality of illumination brings it out more visibly. Sometimes a Leeson disc is used. This consists of three pieces of paper, two thin ones between which a thicker piece, out of which a star is cut, is laid. When equally illuminated on both sides the star appears equally bright on both sides.

The bar photometer is the standard form. A candle or pair of candles may be burned at one end and an incandescent lamp at the other, or a gas flame may first be rated by candles and used as a standard.

Synonyms—Bunsen's Photometer—Translucent Disc Photometer.

Fig. 259. BAR PHOTOMETER.

Photometer. Calorimetric. A photometer in which the radiant energy, so called radiant heat, is used as the measurer of the light.

In one type a differential air thermometer is used, one of whose bulbs is blackened. On exposing this bulb to a source of light it will become heated, and if lights of the same character are used the heating will be in proportion to their illuminating power quite closely. The heating is shown by the movements of the index. By careful calibration the instrument may be made quite reliable.

Photometer, Dispersion. A photometer in which the rays from one of the lights under comparison are made more divergent by a concave lens. In this way a strong light, such as all arc lamp can be photometered more readily than where only the natural divergence of the beam exists. The law of the variation of the intensity of light with the square of the distance is abrogated for a law of more rapid variation by the use of a concave lens.

The diagram, Fig. 260, illustrates the principle. E represents a powerful light, an arc light, to be tested. Its distance from the screen is e. Its light goes through the concave lens L and is dispersed as shown over an area A1, instead of the much smaller area A, which the same rays would otherwise cover. Calling l the distance of the lens from the screen, f its focus, and c the distance of the standard candle from the screen when the shadows are of equal intensity, we have the proportion.

Illuminating power of lamps: ditto of standard candle:: (l (e-l) + fe)2 : (c f)2

413 STANDARD ELECTRICAL DICTIONARY.

Fig. 260. DIAGRAM OF PRINCIPLE OF THE DISPERSION PHOTOMETER.

The cut, Fig. 261, gives a perspective view of Ayrton's Dispersion Photometer. C is the standard candle, L the concave lens, R the rod for producing the two shadows on the screen S.

Fig. 261. AYRTON'S DISPERSION PHOTOMETER.

The mirror M is fixed at an angle of 45 with the stem on which it rotates. The light of the arc lamp is received by the mirror and is reflected through the lens. The candle holder slides along a graduated bar C, and at D is an index plate to show the angle at which the spindle carrying the mirror is set.

414 STANDARD ELECTRICAL DICTIONARY.

Dr. J. Hopkinson in his dispersion photometer uses a double convex lens. This gives a focal image of the arc-lamp between the lens and screen, whence the rays diverge very rapidly, thus giving the desired dispersion effect.

It is principally for arc lamps that dispersion photometers are used.

Photometer, Shadow. A photometer in which the relative intensity of the two lights is estimated by the intensity or strength of shadows of the same object which they respectively cast.

Fig. 262. RUMFORD'S SHADOW PHOTOMETER.

Fig. 263. RUMFORD'S SHADOW PHOTOMETER ARRANGED FOR TESTING INCANDESCENT LAMPS.

415 STANDARD ELECTRICAL DICTIONARY.

A rod is supported in a vertical position. Back of it is a screen of white paper. The two lights to be compared are arranged in front of the rod and at a little distance from each other. They are shifted about until the two shadows appear of equal darkness. The relative intensity of the lights varies inversely with the square of their distances from the shadows cast respectively by them on the screen.

The cut, Fig. 262, shows the simplest type of the shadow photometer. In the cut, Fig. 263, a shadow photometer for testing incandescent lamps is shown. In it E is the lamp under trial supported by a clamp H. A is an ampere meter in circuit with the lamp, and V is a voltmeter. A candle C can be moved along a graduated scale G G. R is the vertical rod, and S is the screen on which the shadows fall.

Photophore. An instrument for medical examination of the cavities of the body. It includes an incandescent lamp mounted in a tube with a concave mirror and convex lens.

Photo-voltaic Effect. The change in resistance of some substances effected by light. Selenium, of all substances, is most susceptible to this effect. (See Selenium.)

Piano, Electric. A piano whose manual or key-board operates to close electric circuits, whereby electro-magnets are caused to operate to drive the hammers against the strings.

Pickle. An acid solution for cleaning metal surfaces before electro-plating, galvanizing or other deposition of metal upon them.

Picture, Electric. A picture produced by passing a strong discharge through a piece of gold leaf clamped or firmly pressed upon a sheet of paper. The gold leaf is cut out of the desired shape, or else a stencil of paper overlays it. The discharge dissipates the gold, and produces a purple colored reproduction of the design upon the paper. The design is due to the deposition of an exceedingly thin film of metallic gold.

Synonym—Electric Portrait.

Pile. A galvanic or voltaic battery. It is sometimes restricted to a number of voltaic couples connected. It should be only applied to batteries with superimposed plates and no containing vessel such as the Dry Pile, q. v., or Volta's Pile, q. v.

Pilot Transformer. In alternating current distribution a small transformer placed at any part of the system and connected to a voltmeter in the central station, to indicate the potential difference of the leads.

Pilot Wires. Wires brought from distant parts of electric light or power mains, and leading to voltmeters at the central station, so that the potential of distant parts of the system can be watched. The wires can be very small, as they have but little current to transmit.

416 STANDARD ELECTRICAL DICTIONARY.

Pistol, Electric. An experimental apparatus for exhibiting the power of electric incandescence or of the electric spark. A tube is mounted with a handle like a pistol. A plug is provided to screw in and out of its side. The plug carries two wires connected on its inner side by a fine platinum wire, or else disconnected but with their ends brought near together to act as terminals for the production of a spark. To use it the tube is filled with a mixture of air and gas, the latter either hydrogen, hydro-carbon or other combustible gas. The tube when full is corked. The wire is heated to incandescence by a current, or a spark is passed from a Leyden jar or other source of electrostatic excitation. The mixture, if properly proportioned, explodes and expels the cork violently.

Fig. 264. ELECTRIC PISTOL.

Pith. A light and soft cellular tissue forming the central core of exogenous trees and plants. In the older parts of the tree the woody tissue often encroaches in and partly obliterates it.

For electrical pith-balls, the pith of the elder, of corn, or, best of all, of sun-flower stems is used.

Pith-balls. Ball made of pith. They are used in the construction of electroscopes and for other experiments in static electricity.

They are cut out with a sharp knife and their shape may be improved by gentle rolling in the hand or between the fingers.

Pivot Suspension. Suspension poising or supporting of an object on a sharp pivot. This is used for the needle in the ordinary compass. A cavity or inverted cup, which may be made of agate, is attached to the middle of the needle which has a hole for its reception. The centre of gravity of the needle comes below the bottom of the cup.

Pivot suspension is not perfect, as it has considerable friction. There is no restitution force, as with torsion filaments.

417 STANDARD ELECTRICAL DICTIONARY.

Plant. The apparatus for commercial manufacturing or technical works. An electric lighting plant includes the boilers, engines and dynamos for producing the current, and the electric mains and subsidiary apparatus.

Plant Electricity. Electricity manifested by plant life. By means of a galvanometer potential differences are found to exist in different parts of trees or fruits. The roots and interior portions are negative, and the flowers, smaller branches and fruit are positive.

In some cases a contraction of the tissue of plants can be produced by an electric current. The sensitive plant and others exhibit this phenomenon, exactly analogous to the action of muscular tissue.

Plate, Arrester. In a lightning arrester the plate connected to the circuit. Sometimes both plates are designated arrester plates.

Plate Condenser. A static condenser having a flat plate of glass for dielectric. (See Epinus' Condenser.)

Plate Electrical Machine. A frictional electric machine, in which a circular plate of glass is excited by friction with the cushions. It is the most recent type of frictional machine and has superseded the old cylinder machines. In its turn it is superseded by influence machines, really plate machines, but not so termed in practice.

Plate, Ground. In a lightning arrester, the plate connected to the earth.

Plate, Negative. In a voltaic battery, either primary or secondary, the plate which is unattacked by the oxygen or negative radical or element of the fluid. It corresponds to the carbon plate in the ordinary voltaic battery, and is the one charged with positive electricity.

Plate, Positive. In a voltaic battery, either primary or secondary, the plate which is dissolved or attacked by the oxygen or negative radical or element of the fluid. It is the plate corresponding to the zinc plate in the ordinary voltaic battery, and is the one charged with negative electricity.

Plating Balance. A balance or scales to which articles in an electroplater's bath are suspended. A weight exceeding by a known amount that of the article as immersed overbalances the article. When the plating is being deposited as soon as it exceeds the excess of weight of the counterpoise the balance tips, the article descends a little, the electric circuit is broken and the plating ceases. Thus the plating is automatically stopped when a predetermined amount of metal is deposited.

418 STANDARD ELECTRICAL DICTIONARY.

Plating Bath. A vessel of solution for the deposition of metal by electrolysis as used in electro-plating.

Plating, Electro-. The deposition of metal by electrolysis so as to coat the conducting surface of objects therewith. The full details of the many processes are very lengthy and cannot be given here.

The general principle includes a battery or source of electric current. The object to be plated is connected to the negative terminal and is immersed in the solution. Thus with a battery the object is in electrical connection with the zinc plate. To the other terminal a metallic plate is connected. The object and the plate termed the anode being introduced into a suitable bath, the metal whose solution is in the bath is deposited upon the surface of the object.

The bath is a solution of the metal in some form that will lend itself to the electrolytic action. The anode is often a plate of the metal of the bath, so that it dissolves as fast as metal is deposited on the object, thus keeping up the strength of the solution.

The objects to be plated must be scrupulously clean, and great care must be taken to keep the bath uncontaminated.

When the object has a non-conducting surface, it is made conducting by being brushed over with plumbago q.v. In addition iron dust is sometimes dusted over it. This acts by precipitating the metal of the bath directly and thus giving a conducting basis for the metal to deposit on. To avoid getting iron in a bath the object may be dipped in copper sulphate solution. This precipitates copper in place of the iron and leaves the article in good shape for silver or other plating.

Electro-plating, if made thick enough, gives a reverse of the article when separated therefrom. A direct copy can be got by a second plating, on the first plating after separation, or a wax impression can be employed.

Under the different metals, formulae for the baths will be found. (See also Quicking— Steeling—Plating Balance.)

Platinoid. An alloy of copper, nickel, zinc in the proportions of German silver with 1 or 2 per cent of tungsten. It is used for resistances. It has a specific resistance (or resistance per centimeter cube) of about 34 microhms. Its percentage variation in resistance per degree C. (1.8 F.) is only about .021 per cent., or less than half that of German silver. This is its most valuable feature.

419 STANDARD ELECTRICAL DICTIONARY.

Platinum. A metal; one of the elements; symbol, Pt; atomic weight, 197.4; equivalent, 49.35; valency, 4; specific gravity, 21.5. It is a conductor of electricity. The following data refer to the annealed metal at 0 C. (32 F.) Relative Resistance (Silver annealed = 1), 6.022 Specific Resistance, 9.057 microhms. Resistance of a wire, (a) 1 foot long, weighing 1 grain, 2.779 ohms. (b) 1 foot long, 1/1000 inch thick, 54.49 " (c) 1 meter long, weighing 1 gram, 1.938 " (d) 1 meter long, 1 millimeter thick, .1153 " Resistance of a 1 inch cube, 3.565 Electro-chemical equivalent (Hydrogen = .0105), 0.5181.

The coefficient of expansion by heat is almost the same as that of glass. It can be passed through holes in glass and the latter can be melted about it so as to hermetically seal its place of passage through the glass. It is used in incandescent lamps for leading-in wires and other similar uses.

Platinum Black. Finely divided platinum. It is made by boiling a solution of platinic chloride with excess of sodium carbonate and a quantity of sugar, until the precipitate is perfectly black and the supernatant liquid is colorless. It seems to possess a great power of occluding oxygen gas. When heated to redness it becomes spongy platinum. The negative plates of a Smee battery are coated with platinum black.

Platinum-silver Alloy. An alloy of 1 part platinum and 2 parts silver, used for resistance coils.

Relative Resistance (silver annealed = 1 ), 16.21 microhms. Specific Resistance at 0C. (32 F.), 24.39 Resistance of a wire, (a) 1 foot long, weighing 1 grain, 4.197 ohms. (b) 1 foot long, 1/1000 inch diameter, 146.70 " (c) 1 meter long weighing 1 gram, 2.924 " (d) 1 meter long, 1 millimeter diameter, 0.3106 " Resistance of a 1 inch cube, 9.603 microhms. Percentage Variation per degree C. (1.8 F.) at about 20 C. (68 F.), 0.031 per cent.

Synonym—Platinum Alloy.

Platinum Sponge. Finely divided platinum obtained by igniting platinum black, q.v., and also by igniting salts of platinum. It has considerable power of condensing or occluding oxygen. It will, if in good condition, set fire to a jet of hydrogen impinging upon it.

Plow. Contact arms projecting downwards from the motors, trucks, or bodies of electric street cars, which enter the underground conduit through the slot and carry contact pieces or brushes, to take the current for driving the motors from the leads within the conduit.

420 STANDARD ELECTRICAL DICTIONARY.

Plcker Tubes. A special form of Geissler tube designed for the production of stratification and for observing the effects produced in the space surrounding the negative electrode.

Plug. (a) A piece of metal with a handle and a somewhat tapered end, used to make connections by insertions between two plates or blocks of metal slightly separated and with grooves to receive it.

(b) A plug or wedge with two metallic faces, insulated from each other with a separate wire connected to each one. It is used in spring-jacks q. v., to introduce a loop in a circuit.

Synonym—Wedge.

Plug. v. To connect by inserting a plug, as in a resistance box.

Fig. 265. PLUGS FOR RESISTANCE COIL BOX.

Fig. 266. PLUG SWITCH.

Plug, Double. A spring-jack plug or wedge with two pairs of insulated faces, one behind the other, so as to simultaneously introduce two loops into a circuit.

Plug, Grid. A piece or mass of lead oxide, inserted into the holes in the lead plates of storage batteries. The holes are often dovetailed or of uneven section to better retain the plugs.

Plug Infinity. In a box-bridge or resistance box, a plug whose removal from between two disconnected discs opens the circuit. All the other discs are connected by resistance coils of various resistance.

Plug Switch. A switch composed of two contact blocks, not touching each other and brought into electrical connection by the insertion of a metallic plug. The latter is usually provided with an insulating handle, and a seat is reamed out for it in the two faces of the contact blocks.

421 STANDARD ELECTRICAL DICTIONARY.

Plumbago. Soft lustrous graphite, a native form of carbon; sometimes chemically purified. It is used in electro-plating to give a conducting surface to non-conducting objects, such as wax moulds. The surface, after coating with plumbago, is sometimes dusted over with iron dust, which precipitates the metal of the bath and starts the plating. It is sometimes plated with copper, silver or gold, and is then termed coppered, silvered, or gilt plumbago. It is gilded by moistening with etherial solution of gold chloride and exposing to the air, and drying and igniting.

Plunger. A movable core which is used in connection with a so-called solenoid coil, to be drawn in when the coil is excited. (See Coil and Plunger.)

Fig. 267 COIL AND PLUNGER WITH SCALES TO SHOW ATTRACTION.

P. O. Abbreviation for Post Office, q.v.

Poggendorf's Solution. An acid depolarizing and exciting fluid for zinc-carbon batteries. The following is its formula: Water, 100 parts; potassium bichromate, 12 parts; concentrated sulphuric acid, 25 parts. All parts by weight. Use cold.

Point, Neutral. (a) On a commutator of a dynamo the points at the ends of the diameter of commutation, or where the brushes rest upon the surface of the commutator, are termed neutral points. At these points there is no generation of potential, they marking the union of currents of opposite direction flowing from the two sides of the armature into the brushes.

(b) In electro-therapeutics, a place in the intra-polar region of a nerve so situated with reference to the kathode and electrode as applied in treatment, that its condition is unaffected.

Synonym—Indifferent Point.

(c) In a magnet the point of no attraction, situated between the two poles, at about an equal distance from each, so as to mark the centre of a magnet of even distribution of polarity.

(d) In thermo-electricity the point of temperature where the thermo-electric powers of two metals are zero; in a diagram the point where the lines representing their thermo-electric relations cross each other; if the metals are arranged in a thermo-electric couple, one end at a temperature a given amount above, the other at a temperature the same amount below the neutral point, no current or potential difference will be produced.

422 STANDARD ELECTRICAL DICTIONARY.

Point, Null. A nodal point in electrical resonators; a point where in a system of waves or oscillations, there is rest, the zero of motion being the resultant of oppositely directed and equal forces. In electrical resonators it is to be sought for in a point symmetrically situated, with reference to the spark gap, or in a pair of points, which pair is symmetrically placed.

The null point in resonators is found by connecting a lead from one of the secondary terminals of an induction coil to different parts of the resonator. The null point is one where the connection does not give rise to any sparks between the micrometer knobs or spark gap, or where the sparks are of diminished size.

The whole is exactly comparable to loops and nodes in a vibrating string or in a Chladni plate as described in treatises on sound and acoustics. (See Resonance, Electrical—Resonator, Electrical.)

Synonym—Nodal Point.

Point Poles. Magnet poles that are virtually points, or of no magnitude. A long thin magnet with little leakage except close to the ends may be supposed to have point poles within itself a short distance back from the ends.

Points, Consequent. In a magnet with consequent poles, the points where such poles are situated.

Points, Corresponding. In bound electrostatic charges the points of equal charges of opposite potentials; the points at opposite extremities of electrostatic lines of force. This definition implies that the bound charges shall be on equal facing areas of conductors, as otherwise the spread or concentration of the lines of force would necessitate the use of areas of size proportionate to the spreading or concentrating of the lines of force. At the same time it may figuratively be applied to these cases, the penetration of the surface by a single line of force including the area fixed by its relation to the surrounding lines.

Points, Isoelectric. In electro-therapeutics, points of equal potential in a circuit.

423 STANDARD ELECTRICAL DICTIONARY.

Points of Derivation. The point where a single conductor branches into two or more conductors, operating or acting in parallel with each other.

Polar Angle. The angle subtended by one of the faces of the pole pieces of the field- magnet of a dynamo or motor. The centre of the circle of the angle lies in the axis of the armature.

Synonym—Angle of Polar Span.

Polar Extension. An addition made of iron to the poles of magnets. Various forms have been experimented with. The pole pieces of dynamo field magnets are polar extensions.

Synonyms—Pole Piece—Polar Tips.

Polarity, Diamagnetic. The induced polarity of diamagnetic substances; it is the reverse of paramagnetic polarity, or of the polarity of iron. A bar of diamagnetic material held parallel with the lines of force in a magnetic field has a like pole induced in the end nearest a given pole of the field magnet, and vice versa. This theory accounts for the repulsion by a magnet of a diamagnetic substance. The existence of this polarity is rather an assumption. It originated with Faraday.

Polarity, Paramagnetic. The induced polarity of paramagnetic substances, such as iron, nickel, or cobalt.

When such a substance is brought into a magnetic field the part nearest a specific pole of a magnet acquires polarity opposite to that of such pole and is thereby attracted.

Another way of expressing it, in which the existence of a pole in or near to the field is not implied, is founded on the conventional direction of lines of force. Where these enter the substance a south pole is formed and where they emerge a north pole is formed.

Such polarity tends always to be established in the direction of greatest length, if the body is free to rotate.

424 STANDARD ELECTRICAL DICTIONARY.

Polarization. (a) The depriving of a voltaic cell of its proper electro-motive force. Polarization may be due to various causes. The solution may become exhausted, as in a Smee battery, when the acid is saturated with zinc and thus a species of polarization follows. But the best definition of polarization restricts it to the development of counter-electro-motive force in the battery by the accumulation of hydrogen on the negative (carbon or copper) plate. To overcome this difficulty many methods are employed. Oxidizing solutions or solids are used, such as solution of chromic acid or powdered manganese dioxide, as in the Bunsen and Leclanch batteries respectively; a roughened surface of platinum black is used, as in the Smee battery; air is blown through the solution to carry off the hydrogen, or the plates themselves are moved about in the solution.

(b) Imparting magnetization to a bar of iron or steel, thus making a permanent magnet, is the polarization of the steel of which it is made. Polarization may be permanent, as in steel, or only temporary, as in soft iron.

(c) The strain upon a dielectric when it separates two oppositely charged surfaces. The secondary discharge of a Leyden jar, and its alteration in volume testify to the strain put upon it by charging.

(d) The alteration of arrangement of the molecules of an electrolyte by a decomposing current. All the molecules are supposed to be arranged with like ends pointing in the same direction, positive ends facing the positively-charged plate and negative ends the negatively-charged one.

(e) The production of counter-electro-motive force in a secondary battery, or in any combination capable of acting as the seat of such counter-electro-motive force. (See Battery, Secondary—Battery, Gas.) The same can be found often in organized cellular tissue such as that of muscles, nerves, or of plants. If a current is passed through this in one direction, it often establishes a polarization or potential difference that is susceptible of giving a return current in the opposite direction when the charging battery is replaced by a conductor.

Polarization Capacity. A voltaic cell in use becomes polarized by its negative plate accumulating hydrogen, or other cause. This gradually gives the plate a positive value, or goes to set up a counter-electro-motive force. The quantity of electricity required to produce the polarization of a battery is termed its Polarization Capacity or Capacity of Polarization.

Polarization of the Medium. The dielectric polarization, q. v., of a dielectric, implying the arrangement of its molecules in chains or filaments; a term due to Faraday. He illustrated it by placing filaments of silk in spirits of turpentine, and introduced into the liquid two conductors. On electrifying one and grounding (or connecting to earth) the other one, the silk filaments arranged themselves in a chain or string connecting the points of the conductors.

Polar Region. That part of the surface of a magnet whence the internal magnetic lines emerge into the air. (S. P. Thompson.) As such lines may emerge from virtually all parts of its surface, the polar regions are indefinite areas, and are properly restricted to the parts whence the lines emerge in greatest quantity.

Polar Span. A proportion of the circle which represents the transverse section of the armature space between the pole pieces of the field magnet in a dynamo or motor; it is the proportion which is filled by the faces of the pole pieces.

425 STANDARD ELECTRICAL DICTIONARY.

Pole, Analogous. The end of a crystal of a pyroelectric substance, such as tourmaline, which end when heated become positively electrified. On reduction of temperature the reverse effect obtains.

Pole, Antilogous. The end of a crystal of a pyroelectric substance, such as tourmaline, which end, while increasing in temperature, becomes negatively electrified. During reduction of its temperature the reverse effect obtains.

Pole Changer. (a) An automatic oscillating or vibrating switch or contact-breaker which in each movement reverses the direction of a current from a battery or other source of current of fixed direction, as such current goes through a conductor.

(b) A switch moved by hand which for each movement effects the above result.

Pole, Negative. (a) In a magnet the south pole; the pole into which the lines of force are assumed to enter from the air or outer circuit.

(b) In a current generator the pole or terminal into which the current is assumed to flow from the external circuit. It is the negatively charged terminal and in the ordinary voltaic battery is the terminal connected to the zinc or positive plate.

Pole Pieces. The terminations of the cores of field or other electro-magnets, or of permanent magnets. These terminations are variously shaped, sometimes being quite large compared to the core proper of the magnet.

They are calculated so as to produce a proper distribution of and direction of the lines of force from pole to pole. As a general rule the active field should be of uniform strength and the pole pieces may be of contour calculated to attain this end.

Pole, Positive. (a) In a magnet the north pole; the pole from which lines of force are assumed to emerge into the air.

(b) In a current generator the pole or terminal whence the current is assumed to issue into the outer circuit. It is the positively charged terminal, and in the ordinary voltaic battery is the terminal connected to the copper or carbon plate, termed the negative plate.

Poles. (a) The terminals of an open electric circuit, at which there necessarily exists a potential difference, produced by the generator or source of electro-motive force in the circuit.

(b) The terminals of an open magnetic circuit; the ends of a magnetized mass of steel, iron or other paramagnetic substance.

(c) The ends in general of any body or mass which show electric or magnetic properties more developed than those of the central sections of the body.

426 STANDARD ELECTRICAL DICTIONARY.

Pole, Salient. In dynamo and motor field magnets, salient poles are those projecting from the base or main body of the field magnet, as distinguished from consequent poles formed by coils wound on the main body itself.

Fig. 268. SALIENT POLES OF FIELD MAGNET.

Poles, Compensating. A device for avoiding the cross-magnetizing effect on the commutator core due to the lead of the brushes. It consists in maintaining a small bar electro-magnet perpendicularly between the pole pieces. This compensates the cross-magnetizing effect.

Poles of Intensity. The locus of highest magnetic force on the earth's surface. One such pole is in Siberia, another is about lat. 52 N., long. 92 W.

[Transcriber's note: 52 N., long. 92 W is about 250 miles Northeast of Winnipeg.]

Poles of Verticity. The magnetic poles of the earth. (See Magnetic Poles.)

Pole Tips. The extreme ends of the expanded poles of a field magnet. In some machines some of the pole tips are made of cast iron, to alter the distribution of the lines of force and resulting magnetic pull upon the armatures. This is done to take off the weight of the armature from its bearings.

Pole, Traveling. A term applied to the poles produced in the action of a rotatory field, whose poles constantly rotate around the circle of the field. (See Field, Rotatory.)

417 STANDARD ELECTRICAL DICTIONARY.

Porous Cup. A cup of pipe clay, unglazed earthenware or other equivalent material used in voltaic cells to keep two liquids separate and yet to permit electrolysis and electrolytic conduction.

They are necessarily only an expedient, as their porous nature permits considerable diffusion, and were they not porous electrolytic action would be impossible.

Synonym—Porous Cell.

Porret's Phenomenon. In electro-physiology, an increase in the diameter of a nerve produced by the positive pole of a voltaic circuit, when placed in contact with the tissue and near to the nerve in question, the other pole being connected to a more or less remote part of the body.

Portelectric Railroad. A railroad worked by solenoidal attraction, the car forming the core of the solenoids. It includes a series of solenoids or hollow coils of copper wire distributed all along the road and inclosing within themselves the track. On this a cylindrical car with pointed ends moves on wheels. Current is supplied to the solenoid in advance of the car, and attracts it. As it advances it breaks the contacts of the attracting solenoid and turns the current into the one next in advance. This operation is repeated as the car advances.

The solenoids are placed close together, each including in the trial track 630 turns of No. 14 copper wire. The car was of wrought iron, 12 feet long, 10 inches in diameter and weighing 500 lbs. It was proposed to employ the system for transportation of mail matter and similar uses.

Position Finder. An instrument for determining the position of objects which are to be fired at from forts. It is designed for use from forts situated on the water.

Fiske's position finder may be thus generally described. On a chart the channel is divided into squares, and the position finder determines the square in which a vessel lies. For each square the direction and elevation of the guns is calculated beforehand. The enemy can therefore be continuously located and fired at, although from smoke or other cause the object may be quite invisible to the gunner.

It comprises two telescopes situated at distant extremities of as long a base line as is obtainable. These telescopes are kept directed upon the object by two observers simultaneously. The observers are in constant telephonic communication. As each telescope moves, it carries a contact over an arc of conducting material. Below each telescope is an arm also moving over an arc of conducting material. These arcs enter into a Wheatstone bridge and are so connected that when the arm and the distant telescope are at the same angle or parallel a balance is obtained. Thus each observer has the power of establishing a balance. A chart is provided for each of them, and over it the arm connected with the distant telescope and an arm or indicator attached to the telescope at that station move so that as long as both telescopes point at the object and each observer maintains the electric balance, the intersection of the arms shows the position on the chart.

The Position Finder is a simplification and amplification of the Range Finder, q. v. In practice the observers may be placed far from the forts, and may telephone their observations thereto. It has been found accurate within one-third of one per cent.

428 STANDARD ELECTRICAL DICTIONARY.

Positive Direction. The direction which lines of force are assumed to take in the air or outer circuit from a positive to a negative region. It applies to electrostatic, to magnetic and to electro-magnetic lines of force.

Positive Electricity. The kind of electricity with which a piece of glass is charged when rubbed with silk; vitreous electricity.

In a galvanic cell the surface of the copper or carbon plate is charged with positive electricity. (See Electrostatic Series.)

According to the single fluid theory positive electrification consists in a surplus of electricity.

[Transcriber's note: "Positive electricity" is a deficiency of electrons.]

Post Office. adj. Many pieces of electric apparatus of English manufacture are thus qualified, indicating that they are of the pattern of the apparatus used by the British Post Office in its telegraph department.

Potential. Potential in general may be treated as an attribute of a point in space, and may express the potential energy which a unit mass would have if placed at that point.

This conception of potential is that of a property attributable to a point in space, such that if a unit mass were placed there the forces acting upon it would supply the force factor of energy, while the body would supply the mass factor. This property is expressible in units, which produce, if the supposed mass is a unit mass, units of work or energy, but potential itself is neither.

Thus taking gravitation, a pound mass on the surface of the earth (assuming it to be a sphere of 4,000 miles radius) would require the expenditure of 21,120,000 foot pounds to remove it to an infinite distance against gravity. The potential of a point in space upon the surface of the earth is therefore negative and is represented by -21,120,000*32.2 foot poundals (32.2 = acceleration of gravity). (See Poundal.) In practice and conventionally all points on the earth's surface are taken as of zero potential.

[Transcriber's note; 21,120,000 foot pounds is about 8 KWh.]

429 STANDARD ELECTRICAL DICTIONARY.

Potential, Absolute. The absolute electrical potential at a point possesses a numerical value and measures the tendency which the existing electric forces would have to drive an electrified particle away from or prevent its approach to the point, if such a particle, one unit in quantity, were brought up to or were situated at that point. It is numerically equal to the number of ergs of work which must be done to bring a positive unit of electricity from a region where there is absolutely no electric force up to the point in question. (Daniell.) Two suppositions are included in this. The region where there is an electric force has to be and only can be at an infinite distance from all electrified bodies. The moving of the particle must take place without any effect upon the distribution of electricity on other particles.

Potential, Constant. Unchanging potential or potential difference.

The ordinary system of incandescent lighting is a constant potential system, an unvarying potential difference being maintained between the two leads, and the current varying according to requirements.

Potential Difference, Electric. If of any two points the absolute potentials are determined, the difference between such two expresses the potential difference. Numerically it expresses the quantity of work which must be done to remove a unit of electricity from one to the other against electric repulsion, or the energy which would be accumulated in moving it the other way.

A positively charged particle is driven towards the point of lower potential. A negatively charged body is driven in the reverse direction.

Potential Difference, Electro-motive. A difference of potential in a circuit, or in part of a circuit, which difference produces or is capable of producing a current, or is due to the flow of such current.

It may be expressed as the fall in potential or the electro-motive force included between any two points on a circuit. The current in an active circuit is due to the total electro-motive force in the circuit. This is distributed through the circuit in proportion to the resistance of its parts. Owing to the distribution of electro-motive force throughout a circuit including the generator, the terminals of a generator on closed circuit may show a difference of potential far lower than the electro-motive force of the generator on closed circuit. Hence potential difference in such a case has been termed available electro-motive force.

Potential, Electric Absolute. The mathematical expression of a property of a point in space, measuring the tendency which existing electric forces would have to drive an electrified unit particle away from or prevent its approach to the point in question, according to whether the point was situated at or was at a distance from the point in question.

Potential is not the power of doing work, although, as it is expressed always with reference to a unit body, it is numerically equal to the number of ergs of work which must be done in order to bring a positive unit of electricity from a region where there is no electric force—which is a region at an infinite distance from all electrified bodies—up to the point in question. This includes the assumption that there is no alteration in the general distribution of electricity on neighboring bodies. (Daniell.)

In practice the earth is arbitrarily taken as of zero electric potential.

430 STANDARD ELECTRICAL DICTIONARY.

Potential, Fall of. The change in potential between any two points on an active circuit. The change in potential due to the maintenance of a current through a conductor.

The fall in potential multiplied by the current gives work or energy units.

The fall of potential in a circuit and its subsequent raising by the action of the generator is illustrated by the diagram of a helix. In it the potential fall in the outer circuit is shown by the descent of the helix. This represents at once the outer circuit and the fall of potential in it. The vertical axis represents the portion of the circuit within the battery or generator in which the potential by the action of the generator is again raised to its original height.

In a circuit of even resistance the potential falls evenly throughout it.

A mechanical illustration of the relation of fall of potential to current is shown in the cut Fig. 269. A vertical wire is supposed to be fixed at its upper end and a lever arm and cord at its lower end, with weight and pulley imparts a torsional strain to it. The dials and indexes show a uniform twisting corresponding to fall of potential. For each unit of length there is a definite loss of twisting, corresponding to fall of potential in a unit of length of a conductor of uniform resistance. The total twisting represents the total potential difference. The weight sustained by the twisting represents the current maintained by the potential difference. For a shorter wire less twisting would be needed to sustain the weight, as in a shorter piece of the conductor less potential difference would be needed to maintain the same current.

Fig. 269. MECHANICAL ILLUSTRATION OF FALL OF POTENTIAL AND CURRENT STRENGTH.

431 STANDARD ELECTRICAL DICTIONARY.

Fig. 270. ILLUSTRATION OF THE FALL AND REDEVELOPMENT OF POTENTIAL IN AN ELECTRIC CIRCUIT.

The fall of potential in a circuit in portions of it is proportional to the resistance of the portions in question. This is shown in the diagram. The narrow lines indicate high and the broad lines low resistance. The fall in different portions is shown as proportional to the resistance of each portion.

Fig. 271. DIAGRAM OF FALL OF POTENTIAL IN A CONDUCTOR OF UNEVEN RESISTANCE.

Potential, Magnetic. The magnetic potential at any point of a magnetic field expresses the work which would be done by the magnetic forces of the field on a positive unit of magnetism as it moves from that point to an infinite distance therefrom. The converse applies to a negative unit.

It is the exact analogue of absolute electric potential.

The potential at any point due to a positive pole m at a distance r is m/r;. that due to a negative pole - m at a distance r' is equal to -m/r';. that due to both is equal to m/r - m/r' or m(1/r - 1/r').

Like electric potential and potential in general, magnetic potential while numerically expressing work or energy is neither, although often defined as such.

432 STANDARD ELECTRICAL DICTIONARY.

Potential, Negative. The reverse of positive potential. (See Potential, Positive.)

Potential, Positive. In general the higher potential. Taking the assumed direction of lines of force, they are assumed to be directed or to move from regions of positive to regions of negative potential. The copper or carbon plate of a voltaic battery is at positive potential compared to the zinc plate.

Potential, Unit of Electric. The arbitrary or conventional potential—or briefly, the potential of a point in an electric field of force—is, numerically, the number of ergs of work necessary to bring a unit of electricity up to the point in question from a region of nominal zero potential—i. e., from the surface of the earth. (Daniell.) This would give the erg as the unit of potential.

Potential, Zero. The potential of the earth is arbitrarily taken as the zero of electric potential.

The theoretical zero is the potential of a point infinitely distant from all electrified bodies.

Fig. 272. DIAGRAM OF POTENTIOMETER CONNECTIONS.

Potentiometer. An arrangement somewhat similar to the Wheatstone Bridge for determining potential difference, or the electro-motive force of a battery. In general principle connection is made so that the cell under trial would send a current in one direction through the galvanometer. Another battery is connected, and in shunt with its circuit the battery under trial and its galvanometer are connected, but so that its current is in opposition. By a graduated wire, like that of a meter bridge, the potential of the main battery shunt can be varied until no current passes. This gives the outline of the method only.

433 STANDARD ELECTRICAL DICTIONARY.

In the cut A B is the graduated potentiometer wire through which a current is passed in the direction of the arrow. E is the battery under trial, placed in opposition to the other current, with a galvanometer next it. Under the conditions shown, if the galvanometer showed no deflection, the E. M. F. of the battery would be to the E. M. F. between the ends of the potentiometer wire, 1 . . . . .10, as 1.5 the distance between the points of connection, A and D of the battery circuit, is to 10, the full length of the potentiometer wire.

Poundal. The British unit of force; the force which acting on a mass of one pound for one second produces an acceleration of one foot.

[Transcriber's note: The force which acting on a mass of one pound produces an acceleration of ONE FOOT PER SECOND PER SECOND.]

Power. Activity; the rate of activity, of doing work, or of expending energy. The practical unit of electric power is the volt-ampere or watt, equal to 1E7 ergs per second. The kilowatt, one thousand watts or volt-amperes, is a frequently adopted unit.

Power, Electric. As energy is the capacity for doing work, electric energy is represented by electricity in motion against a resistance. This possesses a species of inertia, which gives it a species of kinetic energy. To produce such motion, electro- motive force is required. The product of E. M. F. by quantity is therefore electric energy. (See Energy, Electric.)

Generally the rate of energy or power is used. Its dimensions are ( ( (M^.5)*(L^.5) ) / T ) * ( ( (M^.5) *(L^1.5) )/( T^2) ) (intensity or current rate) * (electro-motive force or potential) = (M * (L^2) ) / (T^3), which are the dimensions of rate of work or activity. The practical unit of electric rate of energy or activity is the volt-ampere or watt. By Ohm's law, q. v., we have C = E/R (C = current; E = potential difference or electro-motive force; R = resistance.) The watt by definition = C*E. By substitution from Ohm's formula we deduce for it the following values: ((C^2) * R) and ((E^2) /R). From these three expressions the relations of electric energy to E.M.F., Resistance, and Current can be deduced.

Power of Periodic Current. The rate of energy in a circuit carrying a periodic current. In such a circuit the electro-motive force travels in advance of the current it produces on the circuit. Consequently at phases or intervals where, owing to the alternations of the current, the current is at zero, the electro-motive force may be quite high. At any time the energy rate is the product of the electro-motive force by the amperage. To obtain the power or average rate of energy, the product of the maximum electro-motive force and maximum current must be divided by two and multiplied by the cosine of the angle of lag, which is the angle expressing the difference of phase.

[Transcriber's note; The voltage phase will lead if the load is inductive. The current phase will lead if the load is capacitive. Capacitors or inductors may be introduced into power lines to correct the phase offset introduced by customer loads.]

434 STANDARD ELECTRICAL DICTIONARY.

Pressel. A press-button often contained in a pear-shaped handle, arranged for attachment to the end of a flexible conductor, so as to hang thereby. By pressing the button a bell may be rung, or a distant lamp may be lighted.

Pressure. Force or stress exerted directly against any surface. Its dimensions are force/area or ((M*L)/(T^2)) / (L^2) = M/(L* (T^2)).

Pressure, Electric. Electro-motive force or potential difference; voltage. An expression of metaphorical nature, as the term is not accurate.

Pressure, Electrification by. A crystal of Iceland spar (calcium carbonate) pressed between the fingers becomes positively electrified and remains so for some time. Other minerals act in a similar way. Dissimilar substances pressed together and suddenly separated carry off opposite charges. This is really contact action, not pressure action.

Primary. A term used to designate the inducing coil in an induction coil or transformer; it is probably an abbreviation for primary coil.

Primary Battery. A voltaic cell or battery generating electric energy by direct consumption of material, and not regenerated by an electrolytic process.

The ordinary voltaic cell or galvanic battery is a primary battery.

Prime. vb. To impart the first charge to one of the armatures of a Holtz or other influence machine.

Fig. 273. PRIME CONDUCTOR AND PROOF PLANE.

435 STANDARD ELECTRICAL DICTIONARY.

Prime Conductor. A metal or metal coated sphere or cylinder or other solid with rounded ends mounted on insulating supports and used to collect electricity as generated by a frictional electric machine.

According to whether the prime conductor or the cushions are grounded positive or negative electricity is taken from the ungrounded part. Generally the cushions are grounded, and the prime conductor yields positive electricity.

Probe, Electric. A surgeon's probe, designed to indicate by the closing of an electric circuit the presence of a bullet or metallic body in the body of a patient.

Two insulated wires are carried to the end where their ends are exposed, still insulated from each other. In probing a wound for a bullet if the two ends touch it the circuit is closed and a bell rings. If a bone is touched no such effect is produced. The wires are in circuit with an electric bell and battery.

Projecting Power of a Magnet. The power of projecting its lines of force straight out from the poles. This is really a matter of magnetic power, rather than of shape of the magnet. In electromagnets the custom was followed by making them long to get this effect. Such length was really useful in the regard of getting room for a sufficient number of ampere turns.

436 STANDARD ELECTRICAL DICTIONARY.

Fig. 274. PRONY BRAKE.

Prony Brake. A device for measuring the power applied to a rotating shaft. It consists of a clamping device to be applied more or less rigidly to the shaft or to a pulley upon it. To the clamp is attached a lever carrying a weight. The cut shows a simple arrangement, the shaft A carries a pulley B to which the clamp B1 B2 is applied. The nuts C1 C2 are used for adjustment.

A weight is placed in the pan E attached to the end of the lever D. The weight and clamp are so adjusted that the lever shall stand horizontally as shown by the index E. If we call r the radius of the pulley and F the friction between its surface and the clamp, it is evident that r F, the moment of resistance to the motion of the pulley, is equal to the weight multiplied by its lever arm or to W*R, where W indicates the weight and R the distance of its point of application from the centre of the pulley or r*F = R*W. The work represented by this friction is equal to the distance traveled by the surface of the wheel multiplied by the frictional resistance, or is 2*PI*r*n*F, in which n is the number of turns per minute. But this is equal to 2*PI*R*W. These data being known, the power is directly calculated therefrom in terms of weight and feet per minute.

Proof-plane. A small conductor, usually disc shaped, carried at the end of an insulating handle. It is used to collect electricity by contact, from objects electrostatically charged. The charge it has received is then measured (see Torsion Balance) or otherwise tested. (See Prime Conductor.)

Proof-sphere. A small sphere, coated with gold-leaf or other conductor, and mounted on an insulated handle. It is used instead of a proof-plane, for testing bodies whose curvature is small.

Fig. 275. BOX BRIDGE.

437 STANDARD ELECTRICAL DICTIONARY.

Proportionate Arms. In general terms the arms of a Wheatstone bridge whose proportion has to be known to complete the measurement. There is a different system of naming them. Some designate by this title the two arms in parallel with each other branching at and running from one end of the bridge to the two galvanometer connections. In the cut of the Box Bridge, A C and A B are the proportionate arms. The third arm is then termed the Rheostat arm. (Stewart & Gee.)

Others treat as proportionate arms the two side members of the bridge in parallel with the unknown resistance and third or rheostat arm.

Synonym—Ratio Arms.

Prostration, Electric. Too great exposure to the voltaic arc in its more powerful forms causes symptoms resembling those of sunstroke. The skin is sometimes affected to such a degree as to come off after a few days. The throat, forehead and face suffer pains and the eyes are irritated. These effects only follow exposure to very intense sources of light, or for very long times.

[Transcriber's note: Arcs emit ultraviolet rays.]

Protector, Comb. A lightning arrester, q. v., comprising two toothed plates nearly touching each other.

Protector, Electric. A protective device for guarding the human body against destructive or injurious electric shocks. In one system, Delany's, the wrists and ankles are encircled by conducting bands which by wires running along the arms, back and legs are connected. A discharge it is assumed received by the hands will thus be short circuited around the body and its vital organs. India rubber gloves and shoe soles have also been suggested; the gloves are still used to some extent.

Pull. A switch for closing a circuit when pulled. It is used instead of a push button, q.v., in exposed situations, as its contacts are better protected than those of the ordinary push button.

Pump, Geissler. A form of mercurial air pump. It is used for exhausting Geissler tubes, incandescent lamp bulbs and similar purposes.

Referring to the cut, A is a reservoir of mercury with flexible tube C connected to a tube at its bottom, and raised and lowered by a windlass b, the cord from which passes over a pulley a. When raised the mercury tends to enter the chamber B, through the tube T. An arrangement of stopcocks surmounts this chamber, which arrangement is shown on a larger scale in the three figures X, Y and Z. To fill the bulb B, the cocks are set in the position Z; n is a two way cock and while it permits the escape of air below, it cuts off the tube, rising vertically from it. This tube, d in the full figure connects with a vessel o, pressure gauge p, and tube c, the latter connecting with the object to be exhausted. The bulb B being filled, the cock m is closed, giving the position Y and the vessel A is lowered until it is over 30 inches below B.

438 STANDARD ELECTRICAL DICTIONARY.

This establishes a Torricellian vacuum in B. The cock n is now turned, giving the position X, when air is at once exhausted from the vessel connected to C. This process is repeated until full exhaustion is obtained. In practice the first exhaustion is often effected by a mechanical pump. By closing the cock on the outlet tube c but little air need ever find its way to the chambers o and B.

Fig. 276. GEISSLER AIR PUMP.

439 STANDARD ELECTRICAL DICTIONARY.

Pumping. In incandescent lamps a periodical recurring change in intensity due to bad running of the dynamos, or in arc lamps to bad feeding of the carbons.

Fig. 277. SPRENGEL AIR PUMP.

Pump, Sprengel. A form of mercurial air pump. A simple form is shown in the cut. Mercury is caused to flow from the funnel A, through c d to a vessel B. A side connection x leads to the vessel R to be exhausted. As the mercury passes x it breaks into short columns, and carries air down between them, in this way exhausting the vessel R. In practice it is more complicated. It is said to give a better vacuum than the Sprengel pump, but to be slower in action.

440 STANDARD ELECTRICAL DICTIONARY.

Pump, Swinburne. A form of mechanical air pump for exhausting incandescent lamp bulbs. Referring to the cut, A is a bulb on the upper part of a tube G; above A are two other bulbs C and D. From the upper end a tube runs to the bulb E. Through the cock L, and tube F connection is made with a mechanical air pump. The tube H leads to a drying chamber I, and by the tube J connects with the lamp bulbs or other objects to be exhausted. The tube G enters the bottle B through an airtight stopper, through which a second tube with stopcock K passes. In use a vacuum is produced by the mechanical pumps, exhausting the lamp bulbs to a half inch and drawing up the mercury in G. The bent neck in the bulb E, acts with the bulb as a trap to exclude mercury from F. When the mechanical pumps have produced a vacuum equal to one half inch of mercury, the cock L is closed and K is opened, and air at high pressure enters. This forces the mercury up to the vessel D, half filling it. The high pressure is now removed and the mercury descends. The valve in D closes it as the mercury falls to the level G. Further air from the lamps enters A, and by repetition of the ascent of the mercury, is expelled, through D. The mercury is again lowered, producing a further exhaustion, and the process is repeated as often as necessary.

Fig. 278. SWINBURNE'S AIR PUMP.

Push-Button. A switch for closing a circuit by means of pressure applied to a button. The button is provided with a spring, so that when pushed in and released it springs back. Thus the circuit is closed only as long as the button is pressed. The electric connection may be made by pressing together two flat springs, each connected to one of the wires, or by the stem of the button going between two springs, not in contact, forcing them a little apart to secure good contact, and thereby bridging over the space between them.

441 STANDARD ELECTRICAL DICTIONARY.

Pyro-electricity. A phenomenon by which certain minerals when warmed acquire electrical properties. (Ganot.) The mineral tourmaline exhibits it strongly. It was originally observed in this mineral which was found to first attract and then to repel hot ashes.

The phenomenon lasts while any change of temperature within certain limits is taking place. In the case of tourmaline the range is from about 10 C. (50 F.) to 150 C. (302 F.) Above or below this range it shows no electrification.

The effect of a changing of temperature is to develop poles, one positive and the other negative. As the temperature rises one end is positive and the other negative; as the temperature becomes constant the polarity disappears; as the temperature falls the poles are reversed.

If a piece of tourmaline excited by pyro-electricity is broken, its broken ends develop new poles exactly like a magnet when broken.

The following minerals are pyro-electric: Boracite, topaz, prehnite, zinc silicate, scolezite, axenite. The following compound substances are also so: Cane sugar, sodium- ammonium racemate and potassium tartrate.

The list might be greatly extended.

The phenomenon can be illustrated by sifting through a cotton sieve upon the excited crystal, a mixture of red lead and flowers of sulphur. By the friction of the sifting these become oppositely electrified; the sulphur adheres to the positively electrified end, and the red lead to the negatively electrified end. (See Analogous Pole-Antilogous Pole.)

Pyromagnetic Motor. A motor driven by the alternation of attraction and release of an armature or other moving part, as such part or a section of it is rendered more or less paramagnetic by heat.

Thus imagine a cylinder of nickel at the end of a suspension rod, so mounted that it can swing like a pendulum. A magnet pole is placed to one side to which it is attracted. A flame is placed so as to heat it when in contact with the magnet pole. This destroys its paramagnetism and it swings away from the magnet and out of the flame. It cools, becomes paramagnetic, and as it swings back is reattracted, to be again released as it gets hot enough. This constitutes a simple motor.

A rotary motor may be made on the same lines. Nickel is particularly available as losing its paramagnetic property easily.

442 STANDARD ELECTRICAL DICTIONARY.

Various motors have been constructed on this principle, but none have attained any practical importance. Owing to the low temperature at which it loses its paramagnetic properties nickel is the best metal for paramagnetic motors.

In Edison's motor, between the pole pieces of an electro-magnet a cylinder made up of a bundle of nickel tubes is mounted, so as to be free to rotate. A screen is placed so as to close or obstruct the tubes farthest from the poles. On passing hot air or products of combustion of a fire or gas flame through the tubes, the unscreened ones are heated most and lose their paramagnetism. The screened tubes are then attracted and the armature rotates, bringing other tubes under the screen, which is stationary. Then the attracted tubes are heated while the others cool, and a continuous rotation is the result.

Fig. 279. EDISON'S PYROMAGNETIC MOTOR.

Pyromagnetic Generator. A current generator producing electric energy directly from thermal energy by pyromagnetism.

Edison's pyromagnetic generator has eight electro-magnets, lying on eight radii of a circle, their poles facing inward and their yokes vertical. Only two are shown in the cut. On a horizontal iron disc are mounted eight vertical rolls of corrugated nickel representing armatures. On each armature a coil of wire, insulated from the nickel by asbestus is wound. The coils are all in series, and have eight connections with a commutator as in a drum armature. There are two main divisions to the commutator. Each connects with an insulated collecting ring, and the commutator and collecting rings are mounted on a spindle rotated by power. Below the circle of vertical coils is a horizontal screen, mounted on the spindle and rotating with it.

A source of heat, or a coal stove is directly below the machine and its hot products of combustion pass up through the coils, some of which are screened by the rotating screen. The effect is that the coils are subjecting to induction owing to the change in permeability of the nickel cores, according as they are heated, or as they cool when the screen is interposed. The two commutator segments are in constant relation to the screen, and current is collected therefrom and by the collecting rings is taken to the outside circuit.

443 STANDARD ELECTRICAL DICTIONARY.

Pyromagnetism. The development of new magnetic properties or alteration of magnetic sensibility in a body by heat. Nickel and iron are much affected as regards their paramagnetic power by rise of temperature.

Fig. 280. PYROMAGNETIC GENERATOR.

Pyrometer, Siemens' Electric. An instrument for measuring high temperatures by the variations in electric resistance in a platinum wire exposed to the heat which is to be measured.

Q. Symbol for electric quantity.

Quad. (a) A contraction for quadrant, used as the unit of inductance; the henry.

(b) A contraction for quadruplex in telegraphy.

[Transcriber's note: A modern use of "quad" is a unit of energy equal to 1E15 (one quadrillion) BTU, or 1.055E18 joules. Global energy production in 2004 was 446 quad.]

Quadrant. A length equal to an approximate earth quadrant, equal to 1E9 centimeters. It has been used as the name for the unit of inductance, the henry, q. v.

Synonym—Standard Quadrant.

444 STANDARD ELECTRICAL DICTIONARY.

Quadrant, Legal. The accepted length of the quadrant of the earth, 9.978E8 instead of 1E9 centimeters; or to 9,978 kilometers instead of 10,000 kilometers.

Quadrature. Waves or periodic motions the angle of lag of one of which, with reference to one in advance of it, is 90, are said to be in quadrature with each other.

[Transcriber's note: If the voltage and current of a power line are in quadrature, the power factor is zero (cos(90) = 0) and no real power is delivered to the load.]

Qualitative. Involving the determination only of the presence or absence of a substance or condition, without regard to quantity. Thus a compass held near a wire might determine qualitatively whether a current was passing through the wire, but would not be sufficient to determine its quantity. (See Quantitative.)

Quality of Sound. The distinguishing characteristic of a sound other than its pitch; the timbre.

It is due to the presence with the main or fundamental sound of other minor sounds called overtones, the fundamental note prevailing and the other ones being superimposed upon it. The human voice is very rich in overtones; the telephone reproduces these, thus giving the personal peculiarities of every voice.

Synonym—Timbre.

Quantitative. Involving the determination of quantities. Thus a simple test would indicate that a current was passing through a wire. This would be a qualitative test. If by proper apparatus the exact intensity of the current was determined, it would be a quantitative determination. (See Qualitative.)

Quantity. This term is used to express arrangements of electrical connections for giving the largest quantity of current, as a quantity armature, meaning one wound for low resistance.

A battery is connected in quantity when the cells are all in parallel. It is the arrangement giving the largest current through a very small external resistance.

The term is now virtually obsolete (Daniell); "in surface," "in parallel," or "in multiple arc" is used.

Quantity, Electric. Electricity may be measured as if it were a compressible gas, by determining the potential it produces when stored in a defined recipient. In this way the conception of a species of quantity is reached. It is also measured as the quantity of current passed by a conductor.

Thus a body whose surface is more or less highly charged with electricity, is said to hold a greater or less quantity of electricity.

It may be defined in electrostatic or electro-magnetic terms. (See Quantity, Electrostatic—Quantity, Electro-magnetic.)

445 STANDARD ELECTRICAL DICTIONARY.

Quantity. Electro-magnetic. Quantity is determined electro-magnetically by the measurement of current intensity for a second of time: its dimensions are therefore given by multiplying intensity or current strength by time. The dimensions of intensity are ( (M^.5) * (L^.5) ) / T therefore the dimensions of electro-magnetic quantity are ( ( (M^.5) * (L^.5) ) / T ) * T = ( (M^.5) * (L^.5) )

Quantity, Electro-magnetic, Practical Unit of. The quantity of electricity passed by a unit current in unit time; the quantity passed by one ampere in one second; the coulomb.

It is equal to 3E9 electrostatic absolute units of quantity and to 0.1 of the electro- magnetic absolute unit of quantity.

One coulomb is represented by the deposit of .00111815 gram, or .017253 grain of silver, .00032959 gram, or .005804 grain of copper, .0003392 gram, or .005232 grain of zinc.

If water is decomposed by a current each coulomb is represented by the cubic centimeters of the mixed gases (hydrogen and oxygen) given by the following formula. ( 0.1738 * 76 * (273 + C ) ) / ( h * 273 ) in which C is the temperature of the mixed gases in degree centigrade and h is the pressure in centimeters of mercury column; or by ( 0.01058 * 30 (491 + F - 32) ) / (h * 491 ) for degrees Fahrenheit and inches of barometer.

[Transcriber's note: 6.24150962915265E18 electrons is one coulomb.]

Quantity, Electrostatic. Quantity is determined electro-statically by the repulsion a charge of given quantity exercises upon an identical charge at a known distance. The force evidently varies with the product of the two quantities, and by the law of radiant forces also inversely with the square of the distance. The dimensions given by these considerations is Q * Q/(L*L). This is the force of repulsion. The dimensions of a force are (M * L) /(T^2). Equating these two expressions we have: (Q^2)/(L^2) = (M*L)/(T^2) or Q = ((M^.5)*(L^1.5)) / T which are the dimensions of electrostatic quantity.

Quantity, Meter. An electric meter for determining the quantity of electricity which passes through it, expressible in coulombs or ampere hours. All commercial meters are quantity meters.

446 STANDARD ELECTRICAL DICTIONARY.

Quartz. A mineral, silica, SiO2. It has recently been used by C. V. Boys and since by others in the making of filaments for torsion suspensions. The mineral is melted, while attached to an arrow or other projectile. It is touched to another piece of quartz or some substance to which it adheres and the arrow is fired off from the bow. A very fine filament of surpassingly good qualities for galvanometer suspension filaments is produced.

As a dielectric it is remarkable in possessing but one-ninth the residual capacity of glass.

Quicking. The amalgamating of a surface of a metallic object before silver plating. It secures better adhesion of the deposit. It is executed by dipping the article into a solution of a salt of mercury. A solution of mercuric nitrate 1 part, in water 100 parts, both by weight, is used.

R. (a) Abbreviation and symbol for Reamur, as 10 R., meaning 10 by the Reamur thermometer. (See Reamur Scale.)

(b) Symbol for resistance, as in the expression of Ohm's Law C=E/R. (rho, Greek r) Symbol for specific resistance.

Racing of Motors. The rapid acceleration of speed of a motor when the load upon it is removed. It is quickly checked by counter-electro-motive force. (See Motor, Electric.)

Radian. The angle whose arc is equal in length to the radius; the unit angle.

Radiant Energy. Energy, generally existing in the luminiferous ether, kinetic and exercised in wave transmission, and rendered sensible by conversion of its energy into some other form of energy, such as thermal energy.

If the ether waves are sufficiently short and not too short, they directly affect the optic nerve and are known as light waves; they may be so short as to be inappreciable by the eye, yet possess the power of determining chemical change, when they are known as actinic waves; they may be also so long as to be inappreciable by the eye, when they may be heat-producing waves, or obscure waves.

Other forms of energy may be radiant, as sound energy dispersed by the air, and gravitational energy, whose connection with the ether has not yet been demonstrated.

Radiation. The traveling or motion of ether waves through space.

[Transcriber's note: The modern term corresponding to this definition is photons. The modern concept of radiation also includes particles— neutrons, protons, alpha (helium) and beta (electrons) rays and other exotic items.]

Radicals. A portion of a molecule, possessing a free bond and hence free to combine directly. A radical never can exist alone, but is only hypothetical. An atom is a simple radical, an unsaturated group of atoms is a compound radical.

447 STANDARD ELECTRICAL DICTIONARY.

Radiometer. An instrument consisting of four vanes poised on an axis so as to be free to rotate, and contained in a sealed glass vessel almost perfectly exhausted. The vanes of mica are blackened on one side.

On exposure to light or a source of heat (ether waves) the vanes rotate. The rotation is due to the beating back and forth of air molecules from the surface of the vanes to the inner surface of the glass globe.

Radiometer, Electric. A radiometer in which the motion of the molecules of air necessary for rotation of the vane is produced by electrification and not by heating.

Radio-micrometer. An instrument for detecting radiant energy of heat or light form. It consists of a minute thermopile with its terminals connected by a wire, the whole suspended between the poles of a magnet. A minute quantity of heat produces a current in the thermopile circuit, which, reacted on by the field, produces a deflection. A convex mirror reflecting light is attached so as to move with the thermopile. The instrument is of extraordinary sensitiveness. It responds to .5E-6 of a degree Centigrade or about 1E-6 degree Fahrenheit.

Radiophony. The production of sound by intermittent action of a beam of light upon a body. With possibly a few exceptions all matter may produce sound by radiophouy.

Range Finder. An apparatus for use on shipboard to determine the distance of another ship or object. It is designed for ships of war, to give the range of fire, so as to set the guns at the proper elevation. The general principle involved is the use of the length of the ship if possible, if not of its width, as a base line. Two telescopes are trained upon the object and kept trained continuously thereon. The following describes the Fiske range finder.

The range finder comprises two fairly powerful telescopes, each mounted on a standard, which can be rotated round a vertical axis, corresponding with the center of the large disc shown in the engraving. One-half of the edge of this disc is graduated to 900 on either side of a zero point, and below the graduation is fixed a length of platinum silver wire. This wire only extends to a distance of 81.10 on either side of zero, and is intended to form two arms of a Wheatstone bridge. The sliding contact is carried by the same arm as the telescope standards, so that it moves with the telescope. The two instruments are mounted at a known distance apart on the ship, as shown diagrammatically in the cut. Here A and B are the centers of the two discs, C and D the arms carrying the telescopes, and E and F the platinum silver wires. Suppose the object is at T, such that A B T is a right angle, then AT=AB/sin(ATB).

448 STANDARD ELECTRICAL DICTIONARY.

If the two sectors are coupled up as shown, with a battery, h, and a galvanometer, by the wires, a b and c d, then since the arm, e, on being aligned on the object takes the position c1 while d remains at zero, the Wheatstone bridge formed by these segments and their connections will be out of balance, and a current will flow through the galvanometer, which may be so graduated as to give the range by direct reading, since the current through it will increase with the angle A T B.

Fig. 281. RANGE FINDER.

In general, however, the angle A B T will not be a right angle, but some other angle. In this case AT = AB / sin(A T B) * sin( A B T), and hence it will only be necessary to multiply the range reading on the galvanometer by the sine of the angle A B T, which can be read directly by the observer at B. This multiplication is not difficult, but by suitably arranging his electrical appliances Lieutenant Fiske has succeeded in getting rid of it, so that the reading of the galvanometer always gives the range by direct reading, no matter what the angle at B may be. To explain this, consider the two telescopes shown in the cut in the positions C and D; the whole current then has a certain resistance.

449 STANDARD ELECTRICAL DICTIONARY.

Next suppose them, still remaining parallel, in the positions C1 and D1. The total resistance of the circuit is now less than before, and hence if C1, one of the telescopes, is moved out of parallel to the other, through a certain angle, the current through the galvanometer will be greater than if it were moved through an equal angle out of a parallel when the telescopes were in the positions C and D. The range indicated is, therefore, decreased, and by properly proportioning the various parts it is found that the range can always be read direct from the galvanometer, or in other words the multiplication of A B/sin( A T B ) by sin( A B T ) is to all intents and purposes performed automatically. There is, it is true, a slight theoretical error; but by using a small storage battery and making the contents carefully it is said to be inappreciable. Each telescope is fitted with a telephone receiver and transmitter, so that both observers can without difficulty decide on what point to align their telescopes. It will be seen that it is necessary that the lines of sight of two telescopes should be parallel when the galvanometer indicates no current. It has been proposed to accomplish this by sighting both telescopes on a star near the horizon, which being practically an infinite distance away insures the parallelism of the lines of sight.

Rate Governor. An apparatus for securing a fixed rate of vibration of a vibrating reed. It is applied in simultaneous telegraphy and telephoning over one wire. The principle is that of the regular make and break mechanism, with the feature that the contact is maintained during exactly one-half of the swing of the reed. The contact exists during the farthest half of the swing of the reed away from the attracting pole.

Fig. 282. LANGDON DAVIRS' RATE GOVERNOR.

In the left hand figure of the cut, K is the key for closing the circuit. A is the base for attachment of the reed. V is the contact-spring limited in its play to the right by the screw S. C is the actuating magnet. By tracing the movements of the reed, shown on an exaggerated scale in the three right hand figures, it will be seen that the reed is in electric contact with the spring during about one-half its movement. The time of this connection is adjustable by the screw S.

Synonym—Langdon Davies' Rate Governor or Phonophone.

450 STANDARD ELECTRICAL DICTIONARY.

Ray, Electric. Raia torpedo. The torpedo, a fish having the same power of giving electric shocks as that possessed by the electric eel, q. v. (See also Animal Electricity.)

Fig. 283. TORPEDO OR ELECTRIC RAY

Reaction of Dynamo, Field and Armature. A principle of the dynamo current generator, discovered by Soren Hjorth of Denmark.

When the armature is first rotated it moves in a field due to the residual magnetism of the field magnet core. This field is very weak, and a slight current only is produced. This passing in part or in whole through the field magnet cores slightly strengthens the field, whose increased strength reacts on the armature increasing its current, which again strengthens the field. In this way the current very soon reaches its full strength as due to its speed of rotation.

The operation is sometimes termed building up.

Sometimes, when there is but a trace of residual magnetism, it is very hard to start a dynamo.

Reading Telescope. A telescope for reading the deflections of a reflecting galvanometer.

A long horizontal scale is mounted at a distance from the galvanometer and directly below or above the centre of the scale a telescope is mounted. The telescope is so directed that the mirror of the galvanometer is in its field of view, and the relative positions of mirror, scale and telescope are such that the image of the scale in the galvanometer mirror is seen by the observer looking through the telescope.

Under these conditions it is obvious that the graduation of the scale reflected by the mirror corresponds to the deflection of the galvanometer needle.

The scale may be straight or curved, with the galvanometer in the latter case, at its centre of curvature.

Reamur Scale. A thermometer scale in use in some countries of Continental Europe. The temperature of melting ice is 0; the temperature of condensing steam is 80; the degrees are all equal in length. For conversion to centigrade degrees multiply degrees Reamur by 5/4. For conversion to Fahrenheit degrees multiply by 9/4 and add 32 if above 0 R., and if below subtract 32. Its symbol is R., as 10 R.

451 STANDARD ELECTRICAL DICTIONARY.

Recalescence. A phenomenon occurring during the cooling of a mass of steel, when it suddenly emits heat and grows more luminous for an instant. It is a phase of latent heat, and marks apparently the transition from a non-magnetizable to a magnetiz able condition.

Receiver. In telephony and telegraphy, an instrument for receiving a message as distinguished from one used for sending or transmitting one.

Thus the Bell telephone applied to the ear is a receiver, while the microphone which is spoken into or against is the transmitter.

Receiver, Harmonic. A receiver including an electro-magnet whose armature is an elastic steel reed, vibrating to a particular note. Such a reed responds to a series of impulses succeeding each other with the exact frequency of its own natural vibrations, and does not respond to any other rapid series of impulses. (See Telegraph Harmonic.)

Reciprocal. The reciprocal of a number is the quotient obtained by dividing one by the number. Thus the reciprocal of 8 is 1/8.

Applied to fractions the above operation is carried out by simply inverting the fraction. Thus the reciprocal of 3/4 is 4/3 or 1-1/3.

Record, Telephone. Attempts have been made to produce a record from the vibrations of a telephone disc, which could be interpreted by phonograph or otherwise.

Fig. 284. MORSE RECORDER OR EMBOSSER.

452 STANDARD ELECTRICAL DICTIONARY.

Recorder, Morse. A telegraphic receiving apparatus for recording on a strip of paper the dots and lines forming Morse characters as received over a telegraph line. Its general features are as follows:

A riband or strip of paper is drawn over a roller which is slightly indented around its centre. A stylus or blunt point carried by a vibrating arm nearly touches the paper. The arm normally is motionless and makes no mark on the paper. An armature is carried by the arm and an electro-magnet faces the armature. When a current is passed through the magnet the armature is attracted and the stylus is forced against the paper, depressing it into the groove, thus producing a mark. When the current ceases the stylus is drawn back by a spring.

Fig. 285. INKING ROLLER MECHANISM OF MORSE RECORDER.

In some instruments a small inking roller takes the place of the stylus, and the roller is smooth. The cut, Fig. 285, shows the plan view of the ink-roller mechanism. J is the roller, L is the ink well, Cl is the arm by which it is raised or lowered by the electro-magnet, as in the embosser. S S is the frame of the instrument, and B the arbor to which the arm carrying the armature is secured, projecting to the right. A spring is arranged to rub against the edge of the inking roller and remove the ink from it.

The paper is fed through the apparatus by clockwork. At the present day sound reading has almost entirely replaced the sight reading of the recorder.

Recorder, Siphon. A recording apparatus in which the inked marks are made on a strip of paper, the ink being supplied by a siphon terminating in a capillary orifice.

In the cut N S represents the poles of a powerful electro-magnet. A rectangular coil bb of wire is suspended between the coils. A stationary iron core a intensifies the field. The suspension wire f f 1 has its tension adjusted at h. This wire acts as conductor for the current.

453 STANDARD ELECTRICAL DICTIONARY.

The current is sent in one or the other direction or is cut off in practice to produce the desired oscillations of the coil b b. A glass siphon n l works upon a vertical axis l. One end l is immersed in an ink well m. Its longer end n touches a riband of paper o o. The thread k attached to one side of the coil pulls the siphon back and forth according to the direction of current going through the electro-magnet cores. A spiral spring adjusted by a hand-screw controls the siphon. In operation the siphon is drawn back and forth producing a zigzag line. The upward marks represent dots, the downward ones dashes. Thus the Telegraphic Code can be transmitted on it. To cause the ink to issue properly, electrification by a static machine has been used, when the stylus does not actually touch the paper, but the ink is ejected in a series of dots.

Fig. 286. SIPHON RECORDER.

Reducteur for Ammeter. A resistance arranged as a shunt to diminish the total current passing through an ammeter. It is analogous to a galvanometer shunt. (See Multiplying Power of Shunt.)

Reducteur for Voltmeter. A resistance coil connected in series with a Voltmeter to diminish the current passing through it. Its resistance being known in terms of the resistance of the voltmeter it increases the range of the instrument so that its readings may cover double or more than double their normal range.

Reduction of Ores, Electric. Treatment of ores by the electric furnace (see Furnace, Electric.) The ore mixed with carbon and flux is melted by the combined arc and incandescent effects of the current and the metal separates. In another type the metal is brought into a fusible compound which is electrolyzed while fused in a crucible. Finally processes in which a solution of a salt of the metal is obtained, from which the metal is obtained by electrolysis, may be included. Aluminum is the metal to whose extraction the first described processes are applied.

454 STANDARD ELECTRICAL DICTIONARY.

Refraction, Electric Double. Double refraction induced in some materials by the action of either an electrostatic, magnetic or an electro-magnetic field.

The intensity or degree of refracting power is proportional to the square of the strength of field.

Refreshing Action. In electro-therapeutics the restoration of strength or of nerve force by the use of voltaic alternatives, q. v.

Region, Extra-polar. In electro-therapeutics the area or region of the body remote from the therapeutic electrode.

Region, Polar. In electro-therapeutics the area or region of the body near the therapeutic electrode.

Register, Electric. There are various kinds of electric registers, for registering the movements of watchmen and other service. Contact or press buttons may be distributed through a factory. Each one is connected so that when the circuit is closed thereby a mark is produced by the depression of a pencil upon a sheet or disc of paper by electro-magnetic mechanism. The paper is moved by clockwork, and is graduated into hours. For each push-button a special mark may be made on the paper. The watchman is required to press the button at specified times. This indicates his movements on the paper, and acts as a time detector to show whether he has been attending to his duty.

Register, Telegraphic. A term often applied to telegraph recorders, instruments for producing on paper the characters of the Morse or other alphabet.

Regulation, Constant Current. The regulation of a dynamo so that it shall give a constant current against any resistance in the outer circuits, within practical limits. It is carried out in direct current machines generally by independent regulators embodying a controlling coil with plunger or some equivalent electro-magnetic device inserted in the main circuit and necessarily of low resistance. In some regulators the work of moving the regulator is executed mechanically, but under electrical control; in others the entire work is done by the current.

A typical regulator or governor (Golden's) of the first class comprises two driven friction wheels between which is a driving friction wheel, which can engage with one driven wheel only at once. It is brought into engagement with one or the other by a solenoid and plunger.

455 STANDARD ELECTRICAL DICTIONARY.

As it touches one wheel it turns it in one direction. This moves a sliding contact in one direction so as to increase a resistance. This corresponds to a motion of the plunger in one direction. As the driving wheel moves in the opposite direction by a reverse action it diminishes the resistance. Thus the increase and decrease of resistance correspond to opposite movements of the solenoid plunger, and consequently to opposite variations in the current. The whole is so adjusted that the variations in resistance maintain a constant amperage. The resistance is in the exciting circuit of the dynamo.

In Brush's regulator, which is purely mechanical, a series dynamo is made to give a constant current by introducing across the field magnets a shunt of variable resistance, whose resistance is changed by an electro-magnet, whose coils are in circuit with the main current. Carbon resistance discs are used which the electro-magnet by its attraction for its armature, presses with varying intensity. This alters the resistance, decreasing it as the current increases and the reverse. As the connection is in shunt this action goes to maintain a constant current.

Regulation, Constant Potential. The regulation of constant potential dynamos is executed on the same lines as that of constant current dynamos. If done by a controlling coil, it must for constant potential regulation be wound with fine wire and connected as a shunt for some part of the machine.

Regulation of Dynamos. The regulation of dynamos so that they shall maintain a constant potential difference in the leads of their circuit for multiple arc systems or shall deliver a constant current in series systems. Hence two different systems of regulation are required, (a) constant potential regulation—(b) constant current regulation. The first named is by far the more important, as it concerns multiple arc lighting, which is the system universally used for incandescent lighting.

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