THE MEASUREMENT OF RESISTANCE 199 



If the elimination of a from the result is complete, the 

 balance of the bridge will not be upset when a is greatly in- 

 creased or even made infinity by breaking the connection 

 between X and P. Therefore the test for the proper adjustment 

 of the auxiliary ratio is that the bridge remains in balance with a 

 closed and with a open. In precision measurements it is essential 

 that a be made as low as possible, and less than X. 



Reeves Method for Adjusting the Ratio Arms to Eliminate a. 

 Based on the foregoing, the process of adjustment to eliminate 



a is, with a in place, to adjust the main ratio -^ until the bridge 

 is balanced, then to remove a and rebalance by changing the ratio 

 . This second adjustment will throw out the first, so a must be 



replaced and -VT readjusted and so on until by successive approxi- 

 mations such an adjustment is attained that the balance is main- 

 tained with a either closed or open. This process of successive 

 balances eliminates all questions as to the exact values of m 

 and n and their leads. 



When the elimination of a is complete, 



X = P(Yr- ---} 



\Nc + N L I 



The lead resistances to M and N must be determined and allowed 

 for. 



Wenner Method for Eliminating the Effects of Lead Re- 

 sistances and a. For this method of working the Thomson bridge 

 it is necessary that the slides on the main and auxiliary ratio arms 



be mechanically connected so that the relation-^ = --is always 



maintained. The coils are adjusted with this in view. 



Inspection of formula (20) shows that if in addition the resist- 

 ances of the leads to the ratio coils are adjusted so that 



M C N L = N C M L 

 and 



then 



Y P( c \ 



X P \NJ 



