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circuit (Art. 404), which possesses a high electromotiveforce, can leap the short distance, and in doing so volatilises a small quantity of carbon between the points. Carbon vapour being a partial conductor allows the current to continue to flow across the gap, provided it be not too wide; but as the carbon vapour has a very high

[graphic][merged small]

resistance it becomes intensely heated by the passage of the current, and the carbon points also grow hot. Since, however, solid matter is a better radiator than gaseous matter, the carbon points emit far more light

than the arc itself, though they are not so hot. In the arc the most infusible substances, such as flint and diamond, melt; and metals such as gold and platinum are even vapourised readily in its intense heat. When the arc is produced in the air the carbons slowly burn away by oxidisation. It is observed, also, that particles of carbon are torn away from the + electrode, which becomes hollowed out to a cup-shape, and some of these are deposited on the electrode, which assumes pointed form, as shown in Fig. 137. the arc may vary, according to circumstances, from 0.5 ohm to nearly 100 ohms. It is also found that the arc exerts an opposing electromotive-force of its own, and tends to set up a counter-current.

a

The resistance of

To produce an electric light satisfactorily a minimum electromotive-force of 40-50 volts is necessary; and as the current must be at least from 5 to 10 or more ampères, it is clear that the internal resistance of the battery or generator must be kept small. With weaker currents or smaller electromotive-forces it is impracticable to maintain a steady arc. The internal resistance of the ordinary Daniell's or Leclanché's cells (as used in telegraphy) is too great to render them serviceable for producing electric lights. A battery of 40-60 Grove's cells (Art. 171) is efficient, but will not last more than 2 or 3 hours. A dynamo-electric machine (such as described in Art. 407 to 411), worked by a steam-engine, is the best generator of currents for practical electric lighting. The quantity of light emitted by an electric lamp is disproportionate to the strength of the current ; and is, within certain limits, proportional to the square of the heat developed, or to the fourth power of the strength of the current.

372. Electric Arc Lamps. -Davy employed wood charcoal for electrodes to obtain the arc light. Pencils of hard gas-carbon were later introduced by Foucault. In all the more recent arc lamps, pencils of a more

dense and homogeneous artificial coke-carbon are used. These consume away more regularly, and less rapidly, but still some contrivance is necessary to push the

points of the carbons forward as fast as needed. It is requisite that the mechanism should start the arc by causing the pencils to touch and then separate them to the requisite distance for the production of a steady arc; the mechanism should also cause the carbons not only to be fed into the arc as fast as they consume, but also to approach or recede automatically in case the arc becomes too long or too short; it should further bring the carbons together for an instant to start the arc again if by any chance the arc goes out. Electric Arc Lamps or Regulators, fulfilling these conditions, have been invented by a number of persons. These may be classified as follows:

DUBOSCO

(a) Clockwork Lamps.-Fig. 138 shows the regulator of Foucault as constructed by Duboscq; in this lamp the carbon-holders are propelled by a train of clockwork wheels actuated by a spring. An electromagnet at the base, through which the current runs, attracts an armature and governs the clockwork. If the current is too strong the armature is drawn down, and the clockwork draws the carbons further apart. If the current is weakened by the resistance of the arc, the armature is drawn upwards by a spring, and a second train of wheels comes into play and moves the carbons nearer together.

Fig. 138.

Clockwork arc lamps have also been devised by Serrin and by Crompton, in which the weight of the carbonholders drive the clockwork mechanism.

(b) Break-wheel Lamps.—Jaspar and Crompton have devised mechanism for regulating the rate of feeding the carbon into the arc by adding to the train of wheels a break-wheel; the break which stops the wheel being actuated by a small electromagnet which allows the wheel to run forward a little when the resistance of the arc increases beyond its normal amount.

(c) Solenoid Lamps.—In this class of arc lamp one of the carbons is attached to an iron plunger capable of sliding vertically up or down inside a hollow coil or solenoid, which, being traversed by the current, regulated the position of the carbons and the length of the arc. Siemens employed two solenoids acting against one another differentially, one being a main-circuit coil, the other being a shunt-circuit. If the resistance of the arc became too great, more of the current flowed past the lamp through the shunt-circuit, and caused the carbonholders to bring the carbons nearer together. Shuntcircuits to regulate the arc have also been used by Lontin, Brush, Lever, and others.

(d) Clutch Lamps.—A somewhat simpler device is that of employing a clutch to pick up the upper carbon holder, the lower carbon remaining fixed. In this kind of lamp the clutch is worked by an electromagnet, through which the current passes. If the lamp goes out the magnet releases the clutch, and the upper carbon falls by its own weight and touches the lower carbon. Instantly the current starts round the electromagnet, causes it to act on the clutch which grips the carbonholder and raises it to the requisite distance. Should the arc grow too long the lessening attraction on the clutch permits the carbon-holder to advance a little. Hart, Brush, Weston, and Lever employ clutch lamps. 373. Electric Candles. To obviate the expense

and complication of such regulators, electric candles have

Fig. 139.

been suggested by Jablochkoff, Wilde, and others. Fig. 139 depicts Jablochkoff's candle, consisting of two parallel pencils of hard carbon separated by a thin layer of plaster of Paris and supported in an upright holder. The arc plays

across the summit between the two carbon wicks. In order that both carbons may consume at equal rates, rapidly alternating currents must be employed, which is disadvantageous from an economical point of view.

374. Incandescent Electric Lamps.-Voltaic arcs of an illuminating power of less than 100 candles cannot be maintained steady in practice, and are uneconomical. For small lights it is both simpler and cheaper to employ a thin continuous wire or filament of some infusible conductor, heated to whiteness by passing a current through it. Thin wires of platinum have repeatedly been suggested for this purpose, but they cannot be kept from risk of fusing. Iridium wires and thin strips of carbon have also been suggested by many inventors. Edison in 1878 devised a lamp consisting of a platinum spiral combined with a short-circuiting switch to divert the current from the lamp in case it became overheated. More recently thin filaments of carbon have been employed by Swan, Edison, Lane-Fox, Maxim, Crookes, and others for the construction of little incandescent lamps. In these lamps the carbon filament is mounted upon conducting wires, usually of platinum, which pass into a glass bulb, into

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