56 H. W. Wiley—Carbons in the Electric Lamp. 
the purpose in view. But as the carbons, little by little, be- 
came heated, the copper film oxidized, and after half an hour 
the carbon was again reduced to the slender form above 
described. 
I next tried the expedient of setting a copper wire, -4™™ in 
diameter, into the center of the carbons. With a thin saw I 
copper from oxidation, the copper would prevent the carbon 
illumination. The points of both positive and negative 
_ carbons remained blunt, and there was no wasting away of the 
stem. A carbon prepared in this way will last at least ten 
times as long as one used in the ordinary way. But the chief 
advantage is found in the comparative steadiness of the light 
thus secured. 
Carbons of the above description work best when well 
plated. The following numbers give what I regard as a mini- 
mum amount of copper to secure satisfactory results. In all 
= 
