224 ELECTRIC LIGHTING. 



removed from the parts of the carbon in contact with the 

 insulator. We are told also that in America this metalliza- 

 tion was not deemed sufficient, and the carbons were further 

 covered with plaster to keep the metal from oxidizing ap- 

 parently not without advantage. 



We have seen that to light the Jablochkoff candles the 

 current must first be passed through a secondary conductor. 

 W T hen the arc is once established, this secondary conductor 

 is volatilized, and nothing more remains by which the candles 

 could be lighted again if they went out. This would be an 

 inconvenience if there were not at hand a commutator and 

 candles in reserve. But Jablochkoff has contrived a plan 

 by which there is a secondary conductor between the car- 

 bons when the arc is extinguished. He mixes very fine 

 copper filings with the insulating plaster, and when the latter 

 is volatilized these are also vapourized ; and when the light is 

 extinguished, the copper deposited on the insulator is a suffi- 

 ciently conductive layer to re-light the candle. 



An enamelled glass globe is used with the Jablochkoff 

 candles when these are applied to public lighting, and then 

 the apparatus has the appearance shown in Fig. 65, where 

 part is in section and part in elevation. This arrangement, 

 however, causes a great loss of light, for 45 per cent, is 

 stopped by the globe. It is true the light thus diffused is 

 advantageous for lighting, but these contradictory conditions 

 may be satisfied by using, instead of the globes, double re- 

 flectors : for instance, a kind of funnel of ground glass sur- 

 rounding the candles below and above a hemisphere of 

 translucid glass ; above this should be placed a reflector of 

 enamelled glass or porcelain, which will send down again the 

 luminous rays reflected by the internal surface of the funnel. 

 The electric light globes are, however, now being improved. 

 Paris has made them of opaline glass, absorbing only 35 per 

 cent, of the light. At Baccarat, globes have also been made of 

 frosted glass, which have only a comparatively small absorbing 

 power, and this absorption may be reduced to 10 per cent. 



