554 THE POPULAR SCIENCE MONTHLY. 



so vivid that the sunshine, compared with it, appeared feeble." ' Such 

 effects reached their culmination when, in 1808, through the liberality 

 of a few members of the Royal Institution, Davy was enabled to con- 

 struct a battery of two thousand pairs of plates, with which he after- 

 ward obtained calorific and luminous effects far transcending anything 

 previously observed. The arc of flame between the carbon terminals 

 was four inches long, and by its heat quartz, sapphire, magnesia, and 

 lime were melted like wax in a candle-flame ; while fragments of 

 diamond and plumbago rapidly disappeared, as if reduced to vapor.'' 



The first condition to be fulfilled in the development of heat and 

 light by the electric current is that it shall encounter and overcome 

 resistance. Flowing through a perfect conductor, no matter what the 

 strength of the current might be, neither heat nor light could be de- 

 veloped. A rod of unresisting copper carries away uninjured and 

 unwarmed an atmospheric discharge competent to shiver to splinters a 

 resisting oak. I send the self-same current through a wire composed 

 of alternate lengths of silver and platinum. The silver offers little 

 resistance, the platinum offers much. The consequence is that the 

 platinum is raised to a white heat, while the silver is not visibly 

 warmed. The same holds good with regard to the carbon terminals 

 employed for the production of the electric light. The interval between 

 them offers a powerful resistance to the passage of the current, and it 

 is by the gathering up of the force necessary to burst across this interval 

 that the voltaic current is able to throw the carbon into that state of 

 violent intestine commotion which we call heat, and to which its efful- 

 gence is due. The smallest interval of air usually suffices to stop the 

 current. But when the carbon points are first brought together and 

 then separated, there occurs between them a discharge of incandescent 

 matter which carries, or may carry, the current over a considerable 

 space. The light comes almost wholly from the incandescent carbons. 

 The space between them is filled with a blue flame which, being usually 

 bent by the earth's magnetism, receives the name of the Voltaic Arc. 



For seventy years, then, we have been in possession of this tran- 

 scendent light without applying it to the illumination of our streets 

 and houses. Such applications suggested themselves at the outset, but 

 there were grave difficulties in their way. The first difficulty arose 

 from the waste of the carbons, which are dissipated in part by ordinary 

 combustion, and in part by the electric transfer of matter from the one 

 carbon to the other. To keep the carbons at the proper distance 

 asunder, regulators were devised — the earliest, I believe, by Staite, and 



' Davy, " Chemical Philosophy," p. 110. 



* In the concluding lecture at the Koyal Institution in June, 1810, Davy showed the 

 action of this battery. He then fused iridium, the alloy of indium and osmium, and 

 other refractory substances. See " Philosophical Magazine," vol. xxxv., p. 463. Quetelet 

 assigns the first production of the spark between coal-points to Curtet in 1802. Davy 

 certainly in that year showed the carbon-light with a battery of 150 pairs of plates in 

 the theatre of the Royal Institution (" Journal of the Royal Institution," vol. i., p. 166). 



