Effects of Mechanical Stress on Electrical Resistance. 297 



resistance is not more than 0'7 per cent. This corroborates, for cop- 

 per, very well what Lord Kelvin found in his first experiments on 

 this subject. Since the density of the wire diminishes by O3 per 

 cent., the increase of volume specific resistance is about 0'9 per cent., 

 a result somewhat higher than Mr. Tomlinson's for the copper which 

 he used, as he got 0*6 per cent, for the maximum value. We assume 

 that the latter investigator means " volume specific resistance " by the 

 term " specific resistance " ; otherwise our results would agree more 

 nearly with his. In any case, it seems quite certain that, in copper, 

 the greatest alteration that can be produced by any mechanical treat- 

 ment is not more than would be produced by a rise of temperature of 

 3 Centigrade. 



For additional tests of copper, see end of paper. 



Tests of Soft Iron Wire. 



The wire, 0'8 mm. here used was exceedingly soft, and gave very 

 satisfactory results. This metal is intermediate in properties between 

 copper and steel. Whereas in copper the permanent alteration and 

 stretching begin almost simultaneously with the application of 

 weight, and in steel the alteration is almost altogether temporary, 

 that is, only lasts while the weight is on the wire, in soft iron wire 

 there is both a temporary and permanent alteration. 



The same length of wire was taken as for copper and steel, and 

 weights added by 7 Ibs. each time, tests being made after each 

 addition. A comparison was made of the ratio of the weight specific 

 resistance with weight on to that before any weight was applied. 

 This gives column 3 in the following table, and shows the ratio due 

 to the sum of the temporary and permanent alterations. 



x 2 



