472 



WENNER AND WEIBEL: THE THOMSON BRIDGE 



(3) With the test current supphed through the terminals 

 n' and o', the bridge is balanced by an adjustment of the variable 

 low resistance forming a part of x-t. 



(4) With the test current supplied through the terminals 

 h and 6', the bridge is balanced by an adjustment of the dial- 

 switches of the double ratio-set. 



In the other procedure the test current is supplied through 

 the terminals h and h' only, but means are provided for connecting 

 the terminals n and o, and n' and o' by conductors of low resist- 

 ance. The adjustments are made in 4 steps, of which (1) and (4) 

 are the same as given above. In (2) the terminals n and o are 



Fig. 1. 



connected, while the bridge is balanced by an adjustment of Xi, 

 and in (3) the terminals n' and o' are connected while the bridge 

 is balanced by an adjustment of x^. 



Either procedure gives very accurately 



X = ay/b (1) 



where x and y are the resistances of the four-terminal conductors 

 under comparison; and A and B are the resistances between n 

 and g, and g and o, when the final adjustment is made. 



Both procedures differ from those previously used mainly in 

 that they do not require changing over to a simple bridge by 

 opening the low resistance connection between the resistances 

 under comparison. Where the resistances under comparison 



