[boyle] 



TENSILE STRESS ON ELECTRICAL RESISTANCE 



178 



stant, no matter what the stretching may be . If Z is the original length, 

 and A the original cross-sectional area, 



volume = I A 

 Then any cross- sectional area for a stretch of >Si will be 



i . A 



(Z + S) 



This is about as reasonable an assumption to make as any. In stretch- 

 ing the wire a number of things may happen which we cannot calculate 

 for. For instance ; little cracks, too small to be seen, may develop in the 

 skin of the wire ; one part of the wire may stretch out more than another 

 owing to lack of uniformity in hardness or toughness. 



It would be impossible to measure the changes in diameter by cali- 

 pers; consequently, it is necessary to get a value for the cross-sectional 

 area by making some assumption. 



The weight per unit length of the wtre could have been taken at ths 

 beginning, and the cross-seotion, after any extension, calculated from it. 

 But calculations on this assumption would not include errors due to such 

 causes as mentioned above, though it would have an advantage in avoid- 

 ing errors due to any possible change of density caused by the strain, 

 ■which errors are not included in the other assumption. 



Assuming that» the volume was a constant, and calculating the 

 diameter after the total stretching, and comparing this valuje with the 

 diameter measured by the calipers after all the stretching has taken 

 place, we find the following close agreement: — 



Columns A and B were obtained by taking many readings of the 

 diameter, along the whole length of the wire, on micrometer calipers, and 

 averaging; while the figures in C were calculated on the above assump- 

 tion. It can be seen that, except in the case of Constantin, they agree 



