APPARATUS FOR THE ELECTRIC LIGHT. 129 



has long been objected to, and has often proved an obstacle 

 to the applications of the electric light. Yet even at the 

 present time, and in spite of the progress which has recently 

 been made in the preparation of carbons for the electric 

 light, there are some who prefer the retort-coke. 



The above mentioned inconvenience of retort-carbons, 

 as may readily be supposed, soon caused physicists and 

 manufacturers to seek a method of preparing them of a purer 

 chemical composition, and with a more uniform physical 

 constitution. By certain processes of which we shall pre- 

 sently speak, this has gradually been achieved. Into the 

 composition of these artificial carbons certain metallic salts 

 have sometimes been made to enter, in order that, under the 

 threefold influence of the electrolytic, the calorific, and the 

 reducing actions, the metals of the salts might be deposited 

 on the negative pole, where their combustion in the air would 

 increase the light of the arc itself. Carre, Gauduin, and 

 Archereau have made experiments of this kind, which we 

 shall describe farther on ; but the results have not been 

 entirely satisfactory, as the light has generally been rendered 

 flickering and unsteady. The simple carbon has therefore 

 commonly been preferred. 



The resistance of the carbons is very variable. When 

 they are of uniform section and quality the resistance is, of 

 course, dependent on their length or thickness. Experi- 

 ments made at Silvertown by Hospitalier and Robert Gray 

 have shown a resistance of 3^25 ohms per lineal metre for 

 Carre's carbon of 4 millimetres diameter. A copper con- 

 ductor of the same dimensions would have a resistance of 

 only 0*001315 ohm, and therefore the copper conducts 2,471 

 times better than the carbon. These figures may occasion 

 surprise, but it should be remembered that they relate to 

 carbons of small diameter, and that, if referred to carbon of 

 i centimetre in diameter, the resistance is not more than 

 o'52 ohm, or about 50 metres of telegraph wire. 



As early as the year 1846 Staite and Edwards had patented 



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