76 
MR. H. TOMLINSON - ON THE INFLUENCE OF STRESS 
These results give an alteration of 1160X10 -13 per unit in the resistance for a 
pressure of 1 grm. per square centimetre, and as in the case of copper increase ol 
pressure produces diminution of resistance. 
The experiments were again put an end to by a flaw in the insulation, which was 
made on attempting to carry the pressure to a greater extent, and therefore the final 
result can only be considered as very roughly correct. Fortunately, however, there is 
a means of testing approximately the accuracy of this result, as it was found that at 
the temperature of the room (about 4° C.) the heat caused by the compression of the 
metal exactly balanced the effect of the pressure on the wire—so exactly, indeed, that 
even when pressures of 3000 lbs. per square inch were employed, the instant after the 
removal of the pressure there was no change whatever in the resistance , but in a few 
seconds afterwards the resistance began to decrease, and apparently attained a 
minimum in about half an hour. About this fact there can be no doubt, as it was 
repeated several times and with several different pressures. Now the change of tem¬ 
perature produced by an alteration of fluid pressure amounting to 1 grm. per square 
T x « 
centimetre is for any substance equal to -—-, where T= the absolute temperature, 
a the coefficient of cubical expansion, J Joule’s equivalent, and x the specific heat 
referred to unit volume. In this case T=278, a= ’0000342, J= 42,400, aj='842 at 
the temperature 4° C. 
Therefore the rise of temperature caused by an increase of pressure of 1 grm. per 
square centimetre would be ’000000264° C. ; and since the increase of resistance of 
the wire for 1° C. at 4° C. had by preliminary experiments been determined to be 
almost exactly ’0047 per unit of resistance, the increase of resistance due to the heat 
of compression would be '000000264 X '0047, or 1241 X 10 -12 —a result which agrees 
fairly with that arrived at by direct experiment. 
After making these experiments I was glad to find that Chwolson had succeeded 
in measuring the effect of fluid pressure in the cases of copper, lead, and hard brass,* 
and with these three metals observed that increase of fluid pressure produced decrease 
of electrical resistance. 
In the next table are given the results of Chwolson’s and my own experiments, in 
centimetre gramme units. 
* Im. Acad, of St. Petersburg Bull., March, 1881, and ‘Nature,’ June 2, 1881, p. 112. 
