THE BANISHMENT OF NIGHT 



the carbon in place, the position of this being also deter- 

 mined by an electromagnet. The action is regulated by 

 the difference in the resistance to the passage of the current 

 caused by the increase in the separation of the points. 



In the older type of arc lamp it was necessary to 

 "trim" the lights by replacing the carbons every day; 

 but recently lamps have been perfected in which the 

 carbons last from one hundred to one hundred and 

 twenty hours. In these the arc is enclosed in a glass 

 globe which is made as nearly air-tight as possible with 

 the necessary feed devices. This closed chamber is 

 fitted with a valve opening outward, which allows the 

 air to be forced out by the heat of the lamp, but does not 

 admit a return current. In this manner a rarefied 

 chamber is produced in which the carbons are oxidized 

 very slowly; yet there is no diminution in the brilliancy 

 of the light. 



Early in the history of electric lighting it became ap- 

 parent that the proper construction of the carbon elec- 

 trodes was a highly important item in the manufacture 

 of a lighting apparatus. The value of carbons depends 

 largely upon their purity and freedom from ash in burn- 

 ing, and it required a countless number of experiments 

 to develop the highly efficient carbons now in general 

 use. Davy made use of pieces of wood charcoal in his 

 experiments, but these were too fragile to be of prac- 

 tical value, even if their other qualities had been ideal. 

 Later experimenters tried various compounds, and in 

 1876 Carre* in France produced excellent carbons made 

 of coke, lampblack, and syrup. From these were 

 developed the present carbons, usually made by mixing 



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