570 
MR. R. W. STEWART ON THE ABSOLUTE THERMAL 
several metals, but gave no results bearing on the change of conductivity with tempe¬ 
rature, More recently Kirchhoff and Hansemann^ and LoREXzt have published 
results for iron and copper, but their methods were not so reliable as their work, 
and the results for copper are exceptionally low. 
Although there can be no doubt as to the substantial accuracy of most of the 
results obtained by the experimenters mentioned above, yet it is evident that those 
bearing on the change of conductivity with temperature are very unsatisfactory, and 
that a good deal of work has yet to be done on the subject. 
The method of determination adopted in the work here described was that due to 
Forbes, with the exception that a single thermo-electric couple was employed for the 
determination of temperatures instead of a number of thermometers. In addition to 
the uniformity secured by measuring all temperatures on the same scale, there is the 
advantage that by this method the holes in the bar for the insertion of one junction 
of the couple may be very much smaller than those required for thermometers, and 
may therefore be drilled at shorter distances apart, thus admitting of a more 
accurate determination of the distribution of temperature along the bar. Further, 
as no mercury is put in the holes, no lining of iron is necessary in the copper bar. 
Another advantage in the use of a thermo-electric couple is, that in observing the 
cooling of the short bar used to determine the rate of cooling at different tempera¬ 
tures, the junction inserted in the bar cools at practically the same rate as the bar, 
wliereas, in the case of a thermometer, the coolino- of the thermometer alwavs lasfs 
behind that of the bar;| an experiment showed that at temperatures between 
150° C, and 200° C. the temperature of the thermometer may be nearly a degree 
higher than that of the bar in ’which it is inserted even when this bar is sufficiently 
massive to cool rather slowlv. 
«/ 
The bars used in the experiments were of iron and copper. The iron bar was a 
f-inch square bar, about 4^ feet long, of ordinary commercial wrought-iron, having 
its surface filed up and very lightly polished with black-lead to secure uniformity of 
radiating power. 
The copper bar was a round hard-drawn bar half-an-inch in diameter and about 
7 feet long, of practically pure electrolytically deposited copper, prepared by Messrs. 
Bolton and Co., Oakamoor Mill, Staffordshire, The sectional area of the bar was 
perfectly uniform and its surface was smooth and polished. 
For the reception of the thermo-electric junction, holes about a millimetre in 
diameter were drilled in these bars at distances apart which preliminary experiment 
had shown to be suitable. The depth of the holes was about equal to three-quarters 
of the diameter of the bar, and care was taken in drilling that their axes were at 
right angles to the bar, and passed approximately through its central axis. 
* Wiedemann’s ‘ Annalen,’ 9, p. 1 ; 13, p. 406. 
t Wiedemann’s ‘Annalen,’ 13, p. 422. 
t See FouiiiKR’s ‘Tlieoi’y of Heal,’ §§ 298-300. 
