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XIY. On Thermal Radiation in Absolute Measure. 
By J. T. Bottom ley, M.A. 
Communicated by Sir W. Thomson, Knt., LL.D., F.R.S. 
Received April 23,—Read June 16, 1887. 
[Plates 24-26.] 
In June, 1884,* I liad the honour of laying before the Royal Society a communication 
“ On the Permanent Temperature of Conductors through which an Electric Current 
is passing, and on Surface Emissivity.” # In carrying out the experiments described 
in that communication, it became evident that the method then adopted would lend 
itself readily to the determination in absolute measure of the loss of heat, under 
various circumstances, from the surface of electrically conducting wires ; from metallic 
wires, for example, and from carbon filaments, such as those used in incandescent 
electric lamps. Accordingly, at the conclusion of the paper just referred to, the 
results were given of some preliminary experiments on radiation from metallic wires 
in high vacuums ; and I desire in the present communication to give an account of 
a more extended investigation in the same direction. 
Although loss of heat by radiation and convection has been studied by various 
experimenters, few determinations in absolute measure of the loss under definite 
circumstances have been made ; and, with the exception of those of Schleiermacher, 
to be mentioned immediately, no determinations, so far as I am aware, have been 
made through any considerable range of temperatures, or with a difference of 
temperatures between cooling body and surroundings of considerably more than 
100° C. 
Probably the best known results are those of B. Macfarlane,! who experimented 
on the cooling of polished and blackened copper globes in air, at ordinary pressure 
and saturated with moisture. Professor Tait has also published J the results of 
observations by J. P. Nicol on loss of heat from polished and blackened copper at 
three different pressures, viz., 760 mm. of mercury, 102 mm., and 10 mm. Recently 
* ‘ Roy. Soc. Proc.,’ vol. 37 (No. 232), 1884, p. 177. 
t ‘ Roy. Soc. Proc.,’ vol. 20, 1872. 
J ‘ Edinburgh, Roy. Soc. Proc.,’ 1869-70. 
18.11.87 
