WHAT THE ELECTRIC LI G 117 IS. 13 



part must be traversed by the same quantity of electricity in 

 the same time. 



To obtain, therefore, a very marked calorific effect, we 

 have only to cause the electric discharge to pass through a 

 medium of insufficient conductivity, and this medium may 

 be formed either of a good conductor made very slender, or 

 of a gaseous conductor ; but the luminous effect is always 

 in proportion to the readiness- with which the conductor 

 becomes incandescent. If it is a solid, infusible and badly 

 conducting substances may be used, such as platinum, and 

 especially carbon, provided they have a length and thickness 

 proportional to the electrical intensity of the current. We 

 shall have occasion further on to study some electrical lamps 

 based on this principle. When, on the other hand, the 

 intermediate conductor is gaseous, the electrodes or solid 

 rods conveying the current to it should be such that, while 

 making it capable of conducting the discharge by the eleva- 

 tion of temperature they communicate to it, they may also 

 be able to carry into the midst of this aeriform medium a 

 vast quantity of extremely minute material particles, for the 

 medium derives its luminosity from the glow of red-white 

 heat communicated to these particles. It is known, indeed, 

 that the flame of hydrogen gas, which is of itself not lumi- 

 nous, becomes so when spongy platinum is placed in it, or 

 when the gas is charged with carbon. 



These considerations have shown that carbon should be 

 employed as the means for producing the electric light. 

 This substance is a sufficiently good conductor of electricity; 

 it is easily disintegrated, readily ignited, and being itself 

 combustible, adds the luminous effect of combustion to the 

 brightness of the electric light properly so called. Davy 

 made the first experiments in 1813 on this method of pro- 

 ducing the voltaic arc, and we shall presently see how this 

 discovery was completed by Foucault, who substituted retort 

 carbon for wood charcoal. 



The electric light may also be obtained by means of solid 



