THE MECHANICAL EQUIVALENT OF HEAT. 
391 
external resistance determined for each observation. Also the resistance of the 
platinum wire was observed before and after each determination of the other wires. 
The current was reversed on each occasion to eliminate thermal effects. The total 
swing of the galvanometer averaged about 150 scale divisions for a difference of '01 o), 
and thus the next figure could be determined with accuracy. The actual resistance 
of the wires used varied from 1 to 50 oj; but in the above table the resistance of each 
at 0 ° C. is assumed as 1 w. It must be remembered that the temperatures are 
expressed in the air (not mercury) scale. 
According to the analysis given by Dr. Feuszner, G and H have the same composi¬ 
tion, and differ only in their diameters, yet the difference in their behaviour is very 
marked. It is possible that this difference is due to the effect of the “ drawing.” 
The calculated value of German silver alloy is given for the sake of comparison. 
We regret that we did not include a wire of that substance. It is, of course, improb¬ 
able that its increase is proportional to its temperature—from 0° to 100 ° C. 
Our observations on these wires'^ extended from July 24 to September 18, 1890, 
and we were unable to detect any change in their resistances during that time. 
The behaviour of the alloy C is curious. Our figures, which were carried to a 
place beyond that given in the above table, prove that, over the above range the 
increase in resistance varied strictly as 6. In order to test the straightness of its 
line beyond the above range, we constructed a thermometer which had this wire for 
its coil, but which was, in all other respects, constructed in the same manner as a 
platinum thermometer with double electrodes. 
This thermometer gave Rq = 12 '97 2, = 14 '255, and R., {i.e., resistance in sulphur 
vapour at 760 millims.) = '18 '677. Assuming the straight line we have 
e, = (R, — Ro)/(Ri “ Ro) X 100 = 444-6. 
According to an investigation subsequently conducted as to the boiling point of 
sulphurt the true air temperature ought to be 444°'53 C. It is difficult to imagine a 
closer approximation. A thermometer thus constructed is, therefore, a very simple 
and accurate instrument. It would, however, be exceedingly difficult to make another 
specimen of the alloy of a precisely similar composition, and the above table shows 
that the effect of small changes may be considerable. 
A consideration of the results led to the conclusion that the wire marked H was 
the one most suitable for our purpose. Its change in resistance from 10 ° to 25° C. was 
only 1 in 5000, and since the resistance diminished as 6 increased, it was easy to 
compensate by the addition of a little of the C wire. All difficulties connected with 
change of resistance consequent on the rise of temperature of the water, or of the 
* All these wires had been previously heated to a temperature o£ about 150° G. 
t It must be borne in mind that the date of this determination preceded the work of Callendar and 
Griffiths on this subject, and at this time (July 27, 1890) the boiling point of sulphur was accepted 
as 448°'4 C. (Regnault). 
