32 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



where R is the resistance of the wire, dR is the resistance variation 

 due to the variation of strain de, ju is the Poisson ratio, and p the 

 resistivity. Of the two terms of the right side of Eq. (1), the first 

 denotes the geometric effect, the second the physical effect of resis- 

 tivity variation. Apparently, the change of resistivity has its origin 

 in a variation of the number of free electrons and of their mobility. 1 

 In the strain-gauge engineering literature, the behavior of the 

 strain gauge is commonly expressed by 



AR AL 



~R =S ^7 



(2) 



where L is the length and R the resistance of the unstrained wire, 

 AL its change of length, and AR its change of resistance caused by 

 external stress. The dimensionless magnitude S (sensitivity factor 

 of the wire) can be positive or negative and varies for different metals 

 between — 12.1 and +3.6. Table 4 shows some representative values 

 oiS. 



The resistivity of the different wire materials changes with tem- 

 perature; therefore, temperature variations will cause errors in the 

 use of wire strain gauges. Figures for the temperature-resistivity 

 coefficient are given in column 4 of Table 4. 



The effect of resistance variation with strain is small; the resist- 

 ance of most wire strain gauges varies by not more than 1 per cent 

 for a practically useful range of applied strain. 



1 G. C. Kuczynski, Pkys. Rev., 94, 61 (1954); see this paper for references 

 on recent work concerning the physical mechanism of strain gauges. 



