372 
DR. J. T. BOTTOMLEY AND MR. F. A. KING ON THERMAL 
NOTE .—By Dr. J. T. Bottomley. 
Added July 11, 1907.—The foregoing paper contains an account of experiments on 
radiation of heat at very low temperatures, carried out by Mr. F. A. King and the 
writer of the present note. From the results of these experiments we have calculated 
the thermal emissivity, from a carefully sooted surface, at the temperature stated. 
An attempt is made at the end of the paper to put these results into relation with 
the results of older experiments which I carried out fourteen years ago, with the same 
apparatus, and in precisely the same way, hut at considerably higher temperatures. 
No comparison was, however, made in my former papers on this subject between the 
results of these experiments and what I may call the theoretical laws of thermal 
radiation, nor with the law of Stefan deduced from the old experiments of Dulong 
and Petit. The matter was entirely dealt with from an experimental point of view. 
It seems desirable, now, that I should make an endeavour to compare our experi¬ 
mental results, as far as possible, both with Stefan’s law and with results more 
recently obtained by other experimenters. 
Careful experiments on thermal radiation have been made by Wien and Lummer, 
E. St. John, C. Christiansen, Lummer and Pringsheim, and by F. Kurlbaum ; but 
I find it very difficult to compare their results with my own. All my experiments 
have been made by the method of cooling, which gives directly, in absolute measure, 
the emissivity of the surface of the heated body. Almost all the other experimenters 
have used an indirect method, and have inferred the radiating power of the heated 
body from the indications of a bolometer or special form of thermojunction, determin¬ 
ing the temperature to which this receptor is raised by the presence of the radiating 
body. In a paper by Kurlbaum* the experiments were carried on by means of the 
bolometer. The radiation was from the theoretical “ black body” used by Wien and 
Lummer f, i.e., from the blackened interior of a hollow vessel, heated externally with 
steam, and having an opening in front through which the radiation passes; and the 
bolometer, placed opposite to the opening, receives the radiation. The transition from 
arbitrary to absolute units is difficult. The method is to find the temperature to which 
the bolometer rises, and at which it permanently stands, when at a given distance from the 
heated radiating body, and then to ascertain the amount of electric current which will 
keep the bolometer at this temperature. It seems difficult to put these two observa¬ 
tions into sure relationship with each other, and to obtain the radiating power of the 
black body in absolute measure from experiments on the heat received by the cooler of 
the two bodies ; but it is very interesting to find a close agreement between the results 
obtained by this indirect method and those obtained simply by directly measuring the 
* ‘Wiedemann’s Annalen,’ No. 65, 1898. 
t ‘Wiedemann’s Annalen,’ No. 56, 1895, p. 451. 
