APPARATUS FOR THE ELECTRIC LIGHT. 139 



would be solved much better, at least so far as increased con- 

 ductivity is concerned, by connecting with metallic rods. 

 These rods may be placed either within the carbons themselves 

 or in the electric candles on both sides of the insulator, and 

 thus they may become heated without danger of oxidation. 

 Jablochkoff, who had patented this system of rods as early 

 as November, 1878, finds them advantageous only so far as 

 they lessen the consumption of the electrodes; and if instead 

 of a covering of carbon use is made of a covering of mag- 

 nesia and an iron rod, as in the preparation of these candles^ 

 the consumption might be reduced to one-eighth. But this 

 result is obtained at the expense of the brilliancy of the light 

 produced, which is then reduced to that of six or eight gas- 

 jets. Nevertheless, as there are cases where there is an ad- 

 vantage in increasing the duration of the candle at the cost 

 of the luminous intensity, he has patented this system with 

 the idea of using it in Russia for lighting carriages. 



Effect of heat on the conductivity of the carhons* 



Heat is known to modify the electric conductivity of 

 bodies, diminishing that of metallic conductors, and usually 

 increasing that of substances of mediocre conductivity, 

 whether liquid or solid;* and carbon is precisely one of 

 these. 



According to the researches ot Borgman, the tempera- 

 ture of a carbonaceous substance heated to orange-red de- 

 creases its resistance in the following ratio : 



For wood charcoal 0x30370 between 26 and 260* 



For Dormez anthracite ... 0-00265 20 260* 



For Alibert plumbago ... 0-00082 25 250 



For coke 0-00026 26 275* 



It would seem that even a feeble calorific radiation causes 

 a diminution of resistance in plates of wood charcoal, and 

 that between 100 and 125 degrees the resistance of pine- 

 wood, elm, and ebony charcoals notably varies. 



* See my Paper on the conductivity of mediocre conductors, p. 27. 



