2l8 



ELECTRIC LIGHTING. 



and what are the best dimensions for them. After 

 many trials, preference has been given to plaster of Paris as 

 the insulator, and to Carre's carbons 25 centimetres long 

 and 4 millimetres in diameter. When giving a light equal 

 to from 25 to 40 gas-jets, these candles will last for one hour 

 and a half; but we shall presently see that by a 

 very simple arrangement the light founded on this 

 system may be made to last as long as may be de- 

 sired. A Jablochkoff candle is, then, composed 

 of two insulated Carre carbons <:, d, 25 centimetres 

 long, slightly pointed at their upper extremities, 

 and separated by the insulator already mentioned, 

 provided at its upper end with a thin layer of lamp 

 black for conducting the current when the candle 

 is first lighted. This layer is made of black lead 

 mixed with gum, and the candle is charged by 

 merely dipping its end in the mixture. At first 

 the two carbons were connected by a plumbago 

 point kept inks place by a piece of asbestos paper; 

 the preceding plan is now found to be much 

 simpler. At the lower end the carbons are fitted 

 into copper tubes, by which they are connected 

 with the circuit when the candle is placed in the 

 chandelier ; these tubes are inserted in the piece 

 M (made of a silicate or other suitable substance) 

 so as to prevent the carbons from separating from 

 their insulating partition. Before the manufacture 

 (<Efe of Carre's candles was established on the large 

 FIG. 62. scale, a candle like this cost 75 centimes, a rather 

 high price ; but now that the cost of these carbons 

 is much less, the candles can be made at a cheaper rate 

 perhaps they may some day be had for 20 centimes. We 

 cannot now calculate the cost of the electric light on the 

 data which served for the estimates of the early experiments.* 



* Already the Company sells these candles to private persons at 50 

 centimes. 



