THE MECHA.NICAL EQUIVALENT OF HEAT. 
38 9 
In 1889 we used two coils of 2 and 4 oj. one of German silver and one of platinum, 
both covered with a layer of gutta-percha. 
In 1891 we used a coil of about lOco covered with amber varnish and made of 
copper-manganese-nickel alloy, and also a platinum wire of about the same value. 
This year we have contented ourselves with a platinum wire of about 8‘5 w. 
Since H oc and E is, throughout each experiment, a constant, therefore H oc 1 /R- 
Hence if a coil of large resistance is used, the quantity of heat developed will be 
small, unless E is great. 
It appeared to us desirable to keep the value of E as low as possible, for the greater 
the difference of potential at the ends of the coil, the greater is the loss arising from 
imperfect insulation, &c. On the other hand, if the coll is too short the development 
of heat is too localized, and its even distribution throughout the calorimeter rendered 
more difficult. Our experience in the earlier experiments, as also a consideration of 
the masses of water, &c., led us to the conclusion that a difference of potential equal 
to that of from 3 to 5 Clark cells* and a resistance of 8 to 10 w would balance the 
opposing influences in the manner best adapted for our purpose. 
Platinum would at first sight appear to be an unsuitable metal of which to form 
the coil, for its temperature coefficient is very great, and the element of change thus 
introduced involves much arithmetic in the final calculations. For the same reason 
the error due to its rise in temperature above the surrounding water is likely to be more 
serious than if the coil was formed of an alloy. These considerations carried so much 
weight that we undertook (in the summer of 1890) a series of investigations into the 
resistance of the various copper-manganese-nickel alloys, which had been recently 
introduced by Dr. EeuszNer of Berlin. Some of these alloys were made for us 
by Messrs. Johnson and Matthey, and several specimens were given to us by 
Dr. Eeuszner, to whom we owe our best thanks for his kindness; others were 
obtained from Messrs. Wolff, Berlin. 
The behaviour of these alloys was investigated over a range of from — 20° C. 
to 100° C. The different wires passed down a tube about 4 feet long, together 
, with a platinum wire, cut from the same coil as that from which one of our platinum 
thermometers was made. The resistance of this wire {in situ) was determined in ice 
and steam, and the value of S being known, by its determination when in our 
^ platinum thermometer, the mean temperature of the wires surrounding it could be 
deduced with great accuracy. 
We hoped to proceed with an analysis of the composition of all the wires used but 
time has not been sufficient. 
The results are summarized in the following table. 
* Had we felt certain that the insulation of the wire was perfect, we should have had no hesitation in 
increasing the potential difference, in which case we could have worked with larger masses of water. 
The uncertainty as to the temperature of the wire when thickly coated would, however, more than 
counterbalance any advantage gained by the use of larger masses. 
