AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
35 
the rise of temperature caused by the current was calculated from Koi-ilrausch’s 
formulae,* for iron 
71 = 7 * 0(1 — -000447*--00000052; 3 ), 
and for copper 
n=7i 0 (l —-00052« —*00000028$ 2 ); 
where n 0 and n represents the rigidity at 0° C. and t° C. respectively. In Experi¬ 
ment XXVI. a delicate thermometer was placed near the wire, half-way down the 
helix, and the rise of temperature calculated from the mean of several readings taken 
from time to time during the passage of the current. In Experiments XXIV. and 
XXV. the rise of temperature was determined in the following manner—A current 
of 70° was passed through the copper wire used in Experiment XXIV., and the 
soldered junction of a thermo-element, made of fine German-silver and iron wires, each 
about 2 feet in length, was kept in close contact with the copper wire by placing the 
latter between the two former; the other ends of the wires forming the thermo¬ 
element were connected by means of silk-covered copper wire with the terminals of a 
galvanometer, and after being wrapped in tissue paper and cotton-wool, were placed in 
a clip-stand, which was drawn on one side so as to slightly press the soldered junction 
against the suspended copper wire, a layer of tissue paper having been used to insulate 
the latter from the former. After a short time the deflection of the galvanometer 
became constant, and on immersing the junction of the thermo-element in water, 
it was found necessary to raise the temperature 8'5° C. in order to produce the same 
deflection as before; 8'5° C. was therefore assumed to be the rise of temperature 
which would be caused in the copper wire by the above-mentioned current, and the 
rise of temperature produced by the other currents was calculated from the assump¬ 
tion that the heat generated would be proportional to the square of the current 
strength. In the case of the iron wire it was assumed that the specific resistance 
of the iron would be six times that of the copper, and the rise of temperature was 
calculated accordingly. 
An examination of these last tables shows apparently that the torsional rigidity of 
copper and iron is temporarily decreased by the passage of a powerful current, but is 
very little altered by currents of moderate intensity.! 
Experiment XXIV. also shows that the dipping of the platinum wire into the 
mercury-cup did not appreciably affect the time of vibration. 
* ‘Brit. Encyc.,’ Art. : “ Elasticity,” § 79. 
+ I cannot place so much confidence as I could wish in the results of these particular experiments, as 
far as the decrease of rigidity by powerful currents is concerned, the method employed for estimating 
the elevation of tempei’ature produced by the current being evidently only calculated to give a very rough 
approximation to the true values. I hope, however, to be in a position at some future time to attack the 
question in an entirely different way. 
F 2 
