THE MECHANICAL EQUIVALENT OF HEAT. 
393 
It should be noticed that on the above dates we were constantly passing currents 
through the coil in order to adjust the temperature of the calorimeter during our 
stirring experiments. As, however, there was but little current when ascertaining 
the resistance of the wire at different temperatures, the curves were not appreciably 
altered, and we did not recognize the fact that the rise was a consequence of the 
current. 
On August 27, we performed one of our “J” experiments. We then had 
a current of about half ampere passing through the coil for about 1|- hours. We 
had determined the resistance at the commencement of the experiment, and, on 
repeating our observations at its close, we found that the value of H had risen from 
10'5 5 6 to 10‘5 60. It would thus appear that the rise was a function of the current 
rather than of the time. We continued the series of experiments until September 1, 
when we decided to abandon the use of this alloy.'"" The labour thus expended was 
not altogether lost, for, as will be explained, in the section on the measurement 
of temperature, the results were of considerable value to us. 
Had the rise been a function of the time it would not have been so disastrous, since 
we could have deduced the resistance during each experiment. As, however, the 
change was caused by the current, it was impossible to determine (with sufficient 
accuracy) the actual value of R at each stage of the experiment. It appeared probable 
that electrolysis of some kind took place. If, after a current had been passing 
for some time, the ends of the wires from the coil were connected with the galvano¬ 
meter, a considerable deflection was produced, and it was impossible to determine the 
resistance of the coil for about half an hour after the completion of an experiment, 
owing to the disturbance of the galvanometer thus caused. The effect was apparently 
due to polarization rather than to thermal, or “ Thomson ” effects, and decreased 
regularly until it might be neglected, although the differences of temperature in the 
circuit were maintained unaltered. Much time and energy having already been 
expended in experimenting with these various alloys, we decided to retrace our steps 
and revert to a platinum wire coil. Another consideration which influenced us, was 
that we had, during the past few years, given considerable time and attention to the 
behaviour of platinum wires when used in platinum thermometers, and we considered 
that the experience thus gained of the changes in the resistance of such wires under 
varied conditions, might enable us to proceed with more confidence. 
The smaller the diameter of the wire the less its capacity for heat and, in com¬ 
parison, the greater its cooling surface. The wire selected for this year’s experiments 
was supplied by Messrs. Johnson and Matthey. It had a diameter of '004 in., and 
* Mr. Skinner has recently conducted some experiments with the same coil immersed in parathn 
instead of in water ; and although he has passed far greater currents than any used by us, he was unable 
to trace any signs of consequent increase in R. The increase shown during our experiments was, there¬ 
fore, probably due to some action between the wire (when carrying a current) and the watei’, this is not 
unlikely when we remember that one of the metals present was Mn. 
MDCCCXCIII.—A. 3 E 
