CONDUCTIVITIES OF IRON AND COPPER. 
589 
Table showing the emissive powers of the surfaces of the Iron and Copper Bars. 
The emissivities are given in gramme degrees of heat lost per square centim. of 
surface for each degree Centigrade difference of temperature between the surface and 
the surrounding air. The results are corrected for variation of specific heat with 
temperature. 
Diffei’cnce of Tem¬ 
perature. 
Emissive Power. 
Blackened surface of 
Iron Bar. 
Polished smTace of 
Copper Bar. 
°C. 
20 
0-000275 
0-000220 
40 
313 
253 
60 
348 
280 
80 
378 
297 
100 
405 
315 
120 
429 
333 
140 
454 
346 
160 
476 
360 
180 
374 
On comparison of these results with those published by Mr. D. Macfarlane"^ 
and by Professor TAiT,t for somewhat similar surfaces, it wfill be found that the 
agreement is fairly close. 
The experience gathered in the conduct of the experiments descrilied above goes 
to show that in order to obtain a thoroughly satisfactory determination of thermal 
conductivity and its variation with temperature, the bars emplo 3 md should be 
somewhat massive—not less than an inch in diameter—and their surface should be 
plated (to prevent oxidation) and well polished. Further, to avoid the use of very 
long bars, and to diminish as far as possible the importance of the loss of heat from 
the surface of the bar as an element in the determination. Angstrom’s method should 
be adopted in preference to Forbes’. 
It is noteworthy that with Forbes’ method the influence of expetimental errors 
seems to cause an apparent decrease of conductivity with fall of temperature. A 
trace of this effect is seen in the tables giving the values of k for iron and copper, but 
the reduction of an unsatisfactory set of observations made with the copper bar 
before the use of the trough T T (fig. 1) gave results which were not only much too 
low, but which decreased rapidly with fall of temperature. 
In conclusion the author has to express his thanks to Professor Gray for constant 
help and advice during the progress of the work. 
* ‘ Proc. Roy. Soc.,’ 1872, p. 93, or Everett’s ‘Physical Constants,’ p. 107. 
t ‘ Proc. Roy. Soc. Ediub.,’ 1869-70, p. 207. 
