3 1 6 ELECTRIC LIGHTING. 



ductors, and the manner in which the vacuous vessels are 

 exhausted and sealed. Such lamps will, without damage, 

 bear a certain maximum of current for a more or less pro- 

 longed period, the amount of light depending, of course, 

 upon their electrical resistance and the energy of the current. 

 In operation, their average endurance, or "life," has been 

 stated to be, under favourable circumstances, about 1,000 

 hours. The question of the subdivision of the electric light 

 presents no difficulties with these lamps, and they solve the 

 problem of the application of the light to domestic purposes. 

 Swan's Lam^.The inventor of this lamp has discovered 

 .a method of preparing from cotto^ thread very attenuated 

 filaments of carbon of the tenacity 

 requisite for their sufficiently pro- 

 longed stability and endurance when 

 in use. These extremely thin car- 

 bons are perfectly homogeneous 

 throughout, and are so far from be- 

 coming damaged by use that the 

 effect is to a certain extent an in- 

 crease of their solidity and elasticity. 

 FIG. 76. The arrangement of the lamp, which 



is shown on Fig. 76, is extremely 



simple. The filament of carbon, bent round so as to form 

 a spirally circular loop of about one-fifth of an inch in dia- 

 meter, is enclosed in a glass bulb about two inches in 

 diameter. The extremities of the filament are connected 

 in an ingenious manner to two platinum wires, which pass 

 outwards and either form two small loops, or terminate 

 in binding screws, for connecting with the circuit. These 

 platinum wires are fused into the bulb, and are supported by 

 a piece of glass, which descends internally for a certain dis- 

 tance. The bulb is hermetically sealed, after having been 

 completely exhausted by means of a Sprengel pump. The 

 light yielded by these lamps is mild and steady, with an in- 

 tensity depending, of course, on the current of electricity sent 



