378 
MESSRS. W. E. AYRTON AND H. KILGOUR ON THE 
from the Kew Observatory ; secondly, because if one were making such an investiga¬ 
tion again, one would use as standards of comparison the coils of accurately known 
resistances at high temperatures which can now be purchased, but which we were 
advised three years ago had not then the necessary degree of accuracy. 
IT. Variation of Resistance of Platinum Wires with Temperature. 
We will now return to the main part of the investigation B, and it is to be under¬ 
stood that all temperatures mentioned are the true temperatures as corrected by using 
the results obtained from the comparison of the thermometers with the standard 
thermometer. 
The first point was to determine the variation of resistance with temperature of the 
copper bars BB, BB (fig. 6), and the thick copper wires that went to the Wheatstone’s 
bridge. The total resistance of the bars and wires at 0 C C. was O'0050 ohm, and, as 
the wires were protected by an asbestos screen from the action of the heat, the total 
variation in the resistance of the copper bars and wires combined was very small, 
being only 0'0003 ohm between 0° C. and 325° C. This variation was, therefore, 
almost negligible compared with the variation of the resistance of even the thickest 
platinum wire experimented on, for which the following are the results. 
Table I.—Piece of the Platinum Wire that had been used in the Experiments on 
Emissivity. 
May 10th, 1888. Diameter 14 mils, or 0'356 millim. Length unknown, 
but about 23 centims. 
Temperature. 
Resistance, 
in ohms. 
Temperature. 
Resistance, 
in ohms. 
Temperature. 
Resistance, 
in ohms. 
° C. 
° O. 
° C. 
17-5 
0-262 
72-7 
0-310 
148-3 
0-374 
179 
0-262 
81-8 
0-318 
154-4 
0-382 
239 
0-268 
93-3 
0-328 
170-5 
0-392 
37-5 
0-280 
105-3 
0-338 
182-0 
0-402 
46-8 
0-288 
122-0 
0-352 
194-2 
0-412 
61-1 
0-300 
135-1 
0-362 
Although the exact length of the above wire was unknown, the results are not 
the less useful for giving the law of variation of resistance with temperature, 
for, as pointed out in § HI., “Ptesults of Emissivity Experiments,” the observations 
contained in Tables I. to V. were employed to give the ratios of the resistances of 
any one of the wires at any two temperatures, and not the specific resistance at any 
one temperature. Hence it was unnecessary to know the lengths of the wires used 
m the experiments, the results of which we recorded in Tables I. to V., and the 
lengths where mentioned are only approximate in some cases. 
