14 ELECTRIC LIGHTING. 



badly-conducting bodies rendered incandescent, and we shall 

 have occasion to speak of a system of illumination of this kind, 

 devised by Jablochkoff, who makes use of thin plates of 

 kaolin. But in order to obtain these various results a source 

 of electricity must be available, which not only supplies suffi- 

 cient electricity to produce energetic calorific actions, but pos- 

 sesses a tension sufficiently powerful to overcome the resist- 

 ances offered by the intermediate bodies that are to develop 

 the luminous effects ; and besides this the source of elec- 

 tricity must be suitable to the conditions of the experiment. 

 It is obvious that if the gaseous interval interposed between 

 the conductors of the discharge or current is considerable, it 

 will be necessary for the generator of the electricity to 

 possess tension especially, whilst to produce a powerful 

 effect between two rather large carbons, separated by a 

 narrow interval of air, quantity will especially be needed, 

 since the calorific effects required to make the carbons in- 

 candescent are in proportion to the quantity of electricity 

 produced by the generator. 



These two different effects of electric generators may be 

 readily explained by the way in which they operate in setting- 

 up the action. If the generator has a very great tension, 

 such as occurs with Holtz's machine and the induction 

 machines, the discharge may take place directly between the 

 electrodes, which in a manner serve to determine it, leaping 

 from one to the other ; but as the enormous resistance pre- 

 sented by this gaseous interval very much diminishes the 

 electric intensity, the light so produced is very feeble, and it 

 can be increased only by decreasing the resistance of the 

 gaseous medium, which may be done by rarefying it. The 

 discharge then spreads itself in the vacuous vessel, and if the 

 quantity of electricity be increased by means of a condenser 

 a light of some intensity may be obtained. But this intensity 

 will be considerable only when the discharge carries with it 

 those material particles heated to reddish-whiteness, which, 

 as we have already stated, constitute the whole brilliancy of 



