274 



ELECTEIC LIGHT. 



come by sending the electric current alternately 

 through the two carbon rods, so that the pole 

 which at one moment is positive becomes the 

 next moment negative. The carbons are thus 

 kept of uniform length, their upper extremi- 

 ties are always opposite each other, and the 

 light becomes remarkably steady. 



The candles are inclosed in a globe of opaline 

 glass, which subdues the dazzling brilliancy of 

 the electric arc, and converts it into a pure, soft 

 light, though at the expense of about one half 

 the illuminating power of the naked candle. 

 Each globe contains four candles, only one of 

 which is lighted at a time. As each candle 

 burns about an hour and a half, the four an- 

 swer for an entire evening. As soon as one 

 candle is burned down, the current is switched 

 by an automatic commutator to the next, and 

 so on. 



Each candle in the lantern is mounted in a 

 brass tube securely held upright in a pair of 

 jaws, and is connected with the electric motor 

 by means of a cable of seven tinned copper 

 wires which run down the hollow shaft of the 

 lamp-post and are then carried underground 

 in earthenware drainage-pipes. Fig. V shows 



FIG. 7. 



the globe with its four candles, the switch, 

 and the Gramme dynamo-electric machines 

 from which the electric current is obtained. 



With reference to the motive power con- 

 sumed in producing the Jablochkoff light, it is 

 stated that each separate light requires for its 

 production one horse-power of an engine. In 

 Paris sixteen candles are served by a single 

 Gramme machine, which thus absorbs a mo- 

 e of about sixteen horse-power. For 



by the opal globes which are required to soft- 

 en and diffuse the light. Each Jablochkoff 

 candle representing one horse-power is said to 

 have the photometric value of TOO standard 

 candles, but the globe reduces its effective 

 lighting power to something like 300 candles. 

 Inventions designed for the production of 

 the electric light by incandescence are less nu- 

 merous than those belonging to the class just 

 described, but this mode of lighting has never- 

 theless received much attention, and may yet 

 become a rival of the electric arc. As we 

 have seen, the light is obtained by interposing 

 in the circuit some substance that offers in- 

 creased resistance to the passage of the current. 

 Among the materials used for this purpose 

 are platinum, indium, kaolin, and carbon, each 



FIG. 8. 



*. 



but much of its intensity is lost 



having its peculiar advantages, though car- 



bon has th far been fo d at generally 

 suitable. The earliest form of lamp designed 



