172 Electric Lighting by Incandescence . [February, 
would be flatter in the middle and steeper at the ends, so 
that the “surface integral” of the temperature would be 
higher in the case of the carbon than for the platinum ; 
secondly, because a substance like carbon, which is black 
when illuminated by an extraneous light, gives out more 
light than a bright substance like platinum, at the same tem- 
perature, when sufficiently heated to become self-luminous ; 
a phenomenon easily tried by placing in the flame of a spirit 
lamp a piece of platinum foil on which a black spot has been 
made with Indian ink. 
In the preceding investigation no reference has been made 
to the value of the specific heat of the substance rendered 
incandescent ; and reflection will show that, if a continuous 
light be required, the specific heat can (contrary to what has 
been often said) have nothing to do with the question ; for, 
if the eleCtric current only produces per second as much heat 
as will be lost per second by radiation, conveCtion, and con- 
duction after the body has reached a temperature of iooo°C., 
then it must necessarily reach this temperature, since at all 
lower temperatures it will gain more heat than it loses. 
If, however, instead of a constant light, it were, for any 
purpose, desired that a rapid succession of lightings and 
extinguishing of the lamp should be possible, then, undoubt- 
edly, it would be well to employ, for the incandescent rod, 
a substance having a small specific heat. 
I found it not difficult, in 1873, when designing, at the 
request of the Government, an eleCtric lamp for the use of 
the divers who recovered the property sunk in the French 
Mail steamer Nil off the coast of Japan, to make such thin 
carbon wires as I have spoken of in this paper. The process 
I employed was first to scrape an ordinary carbon rod very 
thin, then fix it to the two electrodes inside a glass globe 
from which the air could be exhausted. A strong eleCtric 
current was now passed through the rod, which was burnt 
to the required degree of thinness by the air pumped through 
the globe. The current was then stopped, the air pump 
out, and replaced by nitrogen, when the globe was sealed up. 
Another plan I employed, on account of the fragile nature 
of very thin carbon, was to use platinum wire coated with 
carbon, and rendered incandescent in an atmosphere of 
nitrogen, but it was difficult to prevent the carbon coming 
off the smooth platinum wire. 
