430 
MR. J. T. BOTTOMLEY ON THERMAL 
A. SchleiermaCHER,,'" using a method precisely similar to one of two which I described 
in the preliminary paper already referred to, has experimented in high vacuum through 
wide ranges of temperature. The object of these experiments was to test the 
supposed law of Stefan, viz., that radiation of heat is in proportion to the fourth 
power of the absolute temperature of the radiating surface. Schleiermacher’s 
results are not stated in absolute measure ; but, from the data given in his paper, I 
have been able to make the calculations, and to compare his results with my own. 
Of other recent experimenters it is only necessary to mention Kundt and 
Warburr, who in 1875 published an important research on friction and heat 
conduction in rarified gases ; t and Mr. Crookes,^ who has given a very valuable 
comparative determination of the loss of heat from the same surface, the bulb of 
a mercury in glass thermometer, at different pressures from full atmospheric pressure 
down to a pressure of two millionths of an atmosphere. 
In order to make this paper complete, it seems advisable that I should here explain 
fully the method of experimenting, and the apparatus used ; even though this has 
already been partially done in my preliminary paper. 
The general principle of my method of experimenting may be thus described : the 
wire for which it is desired to measure the surface loss of heat is stretched in the air 
or so-called vacuum, and an electric current is passed through it. The wire becomes 
heated; but very shortly the temperature becomes steady, a balance having been 
established between the energy supplied by the current and that lost by emission 
from the surface and by conduction to the fixed supports at the extremities. The 
latter part is of but small importance in the fine wires, one metre long, which I have 
been using, and it is allowed for in accordance with a calculation given by Sir William 
Thomson in a-note appended to my first paper. § 
When a permanent temperature has been attained by the wire, the current passing 
is determined in absolute measure. Simultaneously the resistance of the wire is 
measured; and from these measurements the energy is calculated which is expended 
in maintaining the temperature of the wire—that is, as has been said, the energy 
lost at the surface of the wire. The measurement of the electric resistance of the 
wire also enables me to calculate the temperature of the wire, by means of the results 
of a separate determination of the electric resistance at different temperatures of the 
particular wire in use. Lastly, the dimensions of the wire are measured, so that the 
loss of heat per square centimetre may be calculated; and the circuinsfauces of the 
experiment and condition of the surroundings are noted. 
Fig. 1, Plate 24, shows the electric connections of the apparatus chiefly used for 
* ‘Wiedemann, Annalen,’ vol. 26, 1885, p. 287. 
t ‘ Poggendorff, Annalen,’ vols. 155 and 156, 1875. 
X ‘ Roy. Soc. Proc.,’ vol. 21, 1881, p. 239. 
§ ‘Roy. Soc. Proc.,’ vol. 37, 1884, p. 187. 
