ELECTRIC LIGHT.

845 at the binding-screw C, traverses the coil of the electro-magnet E, and passes through the wheel-work to the rack D, which carries the positive carbon. From the positive carbon it passes through the voltaic arc to the negative carbon, and thence, through the support H, to the binding-screw connected with the negative pole of the battery.

When the armature F descends towards the magnet, the other arm of the lever FP is raised, and this movement is resisted by the spiral spring R, which, however, is not attached to the lever in question, but to the end of another lever pressing on its upper side and movable about the point X. The lower side of this lever is curved, so that its point of contact with the first lever changes, giving the spring greater or less leverage according to the strength of the current. In virtue of this arrangement, which is due to Robert Houdin, the armature, instead of being placed in one or the other of two positions, as in some other regulators, has its position continuously varied according to the strength of the current. The anchor Tt is rigidly connected with the lever FP, and follows its oscillations. If the current becomes too weak, the head t moves to the right, stops the fly o' and releases o, which accordingly revolves, and the carbons are moved forward. If the current becomes too strong, o is stopped, o' is released, and the carbons are drawn back. When the anchor Tt is exactly vertical, both flies are arrested, and the carbons remain stationary. The curvature of the lever on which the spring acts being very slight, the oscillations of the armature and anchor are small, and very slight changes in the strength of the current and brilliancy of the light are immediately corrected.

865. Jablochkoff's System of Electric Lighting. The modern revival of interest in the electric light dates from the Paris Exhibition of 1878; when some of the streets of Paris were for the first time lighted by electric lamps constructed on a plan devised by M. Jablochkoff. Instead of placing the two carbons end to end, and providing mechanism for keeping them at the proper distance, he dispenses with mechanism, and places them side by side, with an insulating substance between them, which is gradually consumed. AA (Fig. 591) are the two carbons, separated by a stick of plaster of Paris B. The heat produced by the electric current fuses the plaster of Paris between the points of the carbons, and the fused portion acts as a conductor of high resistance, becoming brightly

incandescent. To light the lamp, a piece of carbon, held by an insulator, is laid across the two carbon points until the light appears,

A

Fig. 591. Jablochkoff

Candle.

and is then removed. The lower ends of the carbons are inserted in copper or brass tubes CC, separated from each other by asbestos; and these tubes are connected by binding-screws with the two wires which convey the current.

When the current employed flows always in the same direction, the positive carbon is made twice as large in section as the negative, because it is consumed about twice as fast. When the current is alternating, which is the preferable arrangement, they are made equal.

The light, when used for street lamps, is surrounded by a globe of opal glass, which serves to diffuse its intensity and prevent dazzling.

The current is furnished by a magneto-electric machine, either an ordinary Gramme machine, which gives a current always in one direction, or a Gramme machine specially modified for giving currents in alternate directions. The machine is driven by a small steam or gas engine of as many horse-power as there are lamps to be supplied; sixteen lamps being sometimes supplied in one circuit by a single machine.

865A. Incandescent Lamps.-Another form of electric light more suitable for domestic purposes, because far steadier and less dazzling,

is now coming into favour. A filament of carbon prepared from bamboo, paper, or some other fibrous material, and about as thick as sewing thread, is inclosed within a vacuous glass vessel, its two ends being attached to wires which pass through the base of the lamp and serve as electrodes. A current of proper strength heats the carbon filament to whiteness, causing it to emit a soft and brilliant light, and the carbon is not consumed, as there is no oxygen to produce combustion. The vacuum must be the most perfect that can be obtained with the best Sprengel pump. One of the best known Fig. 591A.—Swan's Incan- of these lamps is represented in Fig. 591A. The thin black line in the interior represents the carbon filament, which is highly elastic and takes two turns at its upper end.

descent Lamp.

APPENDIX.

ELECTRICAL AND MAGNETIC UNITS.

UNITS AND DERIVED UNITS.

(1.) The numerical value of a concrete quantity is its ratio to a particular unit of the same kind; the selection of this unit being always more or less arbitrary.

(2.) One kind of quantity may, however, be so related to two or more others, as to admit of being specified in terms of units of these other kinds. For example, of the three kinds of quantity, called distance (or length), time, and velocity, any one is capable of being expressed in terms of the other two. Velocity can be specified (as regards amount) by stating the distance passed over in a specified time. Distance can be specified by stating the velocity required for describing it in a specified time, and time can be specified by stating the distance described with a specified velocity.

Force, distance, and work are in like manner three kinds of quantity, of which any two are just sufficient to specify the third.

(3.) Calculation is greatly facilitated by employing as few original or underived units as possible. These should be of kinds admitting of easy and accurate comparison; and all other units should be derived from them by the simplest modes of derivation which are available.

DIMENSIONS.

(4.) Velocity is proportional directly to distance described, and inversely to the time of its description; and is independent of all other elements. This is expressed, by saying that the dimensions distance length time

of velocity are

time

or

Again, if we define the unit of velocity to be that with which unit distance would be described in unit time, the real magnitude of the unit of velocity will depend upon the units of length and time selected, being proportional directly to the real magnitude of the former, and inversely to the real magnitude of the latter. This is expressed by saying that the dimensions of the unit of velocity are Jength Both forms of expression are convenient; and the ideas which

time

they are intended to express are logically equivalent.

MECHANICAL UNITS.

(5.) All electrical and magnetic units can be derived from units of length, mass, and time. We shall denote length by l, mass by m, and time by t.

(6.) The unit of velocity is the velocity with which unit length is described in unit time. Its dimensions are t'

(7.) The unit of acceleration is the acceleration which gives velocity unit increase of velocity in unit time. Its dimensions are time

(8.) The unit force is that which acting on unit mass produces unit acceleration. Its dimensions are mass x acceleration, or m

ml

t2.

(9.) The unit of work is the work done by unit force working through unit length. Its dimensions are force × length, or ť2

m 12

mr.

(10.) The unit of kinetic energy is the kinetic energy of two units of mass moving with unit velocity (according to the formula 1⁄2 m v2). Its dimensions are mass x (velocity)2, or and are the same as

m12

12,

the dimensions of work. It might appear simpler to make it the energy of one unit of mass moving with unit velocity; but if this change were made, it would be necessary either to halve the unit of work, or else to make kinetic energy double of the work which produced it. Either of these alternatives would involve greater inconvenience and complexity than the selection made above.

ELECTRO-STATIC UNITS.

(11.) Let q denote quantity of electricity measured statically, so that the mutual repulsion of two equal quantities q at distance l,

ELECTRICAL AND MAGNETIC UNITS.

849

is 2. This being equal to a force, the dimensions of q2 must be

m3, and the dimensions of q must be m1??

13 t2,

(length)2 × force, or (12.) Let V denote difference of potential. Then the work required to raise a quantity q through a difference of potential V, is q V. The dimensions of V are therefore ml. The dimensions of potential arc of course the same as those

of difference of potential.

work

or

m12 t
12 mil

or

(13.) The capacity of a conductor is the quotient of the quantity of electricity with which it is charged, by the potential which this charge produces in the conductor. The dimensions of capacity are therefore or simply l. In fact, as we have seen (§ 613), the capacity of a spherical conductor is equal to its radius.

m 1 3
t

ELECTRO-MAGNETIC UNITS.

(14.) Let P denote the numerical value of a pole (or the strength of a pole). Then, since two equal poles P at distance 7 repel each

P2

other with the force which must be of the dimensions

12,

(15.) Let I denote the intensity of a magnetic field.

pole P in this field is acted on with a force PI.

Then, a This must be of

(16.) Let M denote the moment of a magnet. Since it is the product of the strength of a pole by the distance between two poles, its

(17.) Intensity of magnetization is the quotient of moment by volume. Its dimensions are therefore or t These are the same

as the dimensions of intensity of field.

(18.) When a magnetic substance is placed in a magnetic field, it is magnetized by induction; and each substance has its own specific coefficient of magnetic induction (constant, or nearly so, when the field is not excessively intense), which expresses the ratio of the intensity of the induced magnetization to the intensity of the