37o 



COMPARISON OF RESISTANCES. 



and would give 



If the resistances r lt r^... r n , without being equal, are merely 

 near each other, then, if r is their mean value, and fi lt /3 2 , and fi n 

 very small quantities the sum of which is null, 



neglecting quantities of the order of /3 2 , it follows that 



ii in/ -57A n 



r l + 7 t + -*7--r\ 1 -Trr' 



and therefore 



When the square of the corrections p may be neglected, it is not 

 necessary to determine them. 



Fig. 193- 



972. CALIBRATION OF THE WIRE. The exactitude of the pre- 

 ceding methods assumes the perfect homogeneity of the wire, if 

 not throughout its entire length, at any rate in the part used for 

 the readings. This condition is usually realised when the wire 

 has been carefully prepared with good alloys and drawn with care, 

 but it is necessary to control it by calibration. 



The first method consists in reproducing the same operations as 

 for the calibration of a thermometer tube, and for the construction 

 of the standard (921). An auxiliary resistance is taken which is 

 equivalent to the nth part of the wire, and it is measured by 

 substitution, the lateral resistances being so chosen as to utilise 

 the n parts of the wire. An auxiliary resistance is then used 



