Effects of Mechanical Stress on Electrical Resistance. 293 



But, since the weight of the length between the two fixed marks, 

 R and M, on the wire, remains constant, we have 



weight = 



tv'p LJ 



- - f i j 



wp Jj 



i T i 



ff to :zz ^ nP 



a w a v p 



ta p % 

 wp 



* ~ ~~ ~jf T~> * 



a w I Li 



Thus, when a balance has been found, it is only necessary to- 

 measure, by means of the scales Si S, the lengths I' and L', the other 

 two, I and L, having been read before the weights were applied. The 

 ratio of the weight resistances after and before stretching is thus 

 obtained. If the ratio of the volume resistances is required, we have 

 <r' w la w = ff' v p/ff v p, so that, by cutting off suitable lengths of the wire 

 and determining their densities, we get <//<r. 



The preliminary trials of this method were made on copper wire 

 with the help of Mr. Hamilton Wingate. The results obtained quite 

 agreed with what Lord Kelvin first, and Mr. Tomlinson afterwards,. 

 found, that the mechanical treatment did not materially affect the 

 specific resistance. 



Tests of Steel Wire. 



Pianoforte steel wire, of diameter 0*8 mm., was used, and 

 straightened by a weight of 14 Ibs. An additional 42 Ibs. weight was 



