530 Carbons in the Electric Lamp . [August, 
vent the heating and consequent rapid consumption of the 
electrode. In placing the carbons in the lamp the grooved 
sides were turned from the lens. This device proved very 
successful in securing a steady light, but the carbons were 
heated to their points of insertion in the holders, and wasted 
rapidly away. The point, it is true, remained blunt, but 
the stem of the carbon burned away so rapidly that the 
experiment must be regarded as unsuccessful. 
It was evident from my first experiments with copper- 
plated carbons, that, if any way could be devised of pro- 
tecting the copper from oxidation, the copper would prevent 
the carbon from heating by increasing its conductivity and 
diminishing the resistance. I accordingly covered the car- 
bons, after copper-plating, with a film of plaster of Paris, 
about i millimetre in thickness. After this had set, the 
edge of the carbon, which was to be turned towards the 
condenser, was carefully denuded of its plaster covering, 
which was also cut away until quite thin on the adjacent 
surfaces. These precautions were taken so that the plaster 
might not interfere with the radiation of light in the direc- 
tion of the condenser. Before use the plaster must be 
thoroughly and slowly dried. The copper surface at the 
end of the carbon being uncovered, it is fastened in the 
holder in the usual way. The carbons prepared in the way 
described left nothing to be desired. The light was steady, 
and the carbons burned slowly away. The film of plaster 
melted gradually as the point was consumed, and thus never 
interfered with the 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 ordi- 
nary way. But the chief advantage is found in the compa- 
rative 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 
minimum amount of copper to secure satisfactory results. 
In all the experiments I have tried the carbon has been of 
the soft American variety, with an average sp ecific gravity 
of I’55- 
Length of carbon 
Each side 
Number ^ of surface, including ends 
Weight before coppering 
,, after coppering 
,, of copper ... 
)) )f to each c.m. ••• ••• 
17*5 c.m. 
i 
72 c.m.* 
21-1615 g. 
24-0410 ,, 
2-8795 „ 
0-0397 » 
