128 ELECTRIC LIGHTING. 



APPARATUS FOR EVOLVING THE ELECTRIC LIGHT. 



Light produced between carbon electrodes. We 



saw at the beginning of this work that the electric light was 

 produced by the passage of an electric discharge or current 

 through a gaseous or solid body having a conductivity suffi- 

 ciently small to become incandescent from the enormous 

 heat developed by the passage of the electricity. We saw 

 also that in order to obtain this light under the most favour 

 able conditions it is necessary to use electrodes made of sub- 

 stances capable of disintegration, and of ready combustibility, 

 and that of all substances carbon is that which yielded the 

 best results. 



It was Davy who first conceived the idea of using carbons 

 as electrodes for producing the voltaic arc, but these carbons 

 were sticks of charcoal quenched in water. These were, 

 however, so quickly consumed that other physicists endea- 

 voured to substitute a more durable form of carbon for the 

 wood charcoal. Foucault was the first to make use of that 

 product of coal which is deposited in the inside of the retorts 

 employed in gas-making. He thus obtained a voltaic arc 

 of much greater durability. 



There was, however, much that was objectionable in retort- 

 coke, from its want of uniformity and from its admixture with 

 earthy matters, which caused the light produced to be far 

 from steady. The carbons were destroyed also in conse- 

 quence of the fusion of the siliceous matter contained in 

 them, and generally emitted vapours which, being better 

 conductors than the arc, carried off a portion of the current 

 as a non-luminous discharge. 



It is true that by suitably choosing these carbons, and by 

 cutting them from the uniform portions of the deposits, good 

 ones might be obtained ; but this material for the voltaic arc 



