THE MEASUREMENT OF RESISTANCE 159 



An alternative method is to unplug a small resistance, R*, and 

 obtain a balance by adjusting R 8 , then to make R x large and 

 repeat the balance. If all other resistances remain constant, and 

 the values unplugged be R x , R' x , and R s , R' 8} 



R'x Rx 



X = S 



R' 8 - R 8 



The differential galvanometer was formerly in quite common 

 use but was supplanted by the Wheatstone bridge. Of late years, 

 however, the instrument has again come into use for measure- 

 ments where the range to be covered is not large, for instance in 

 resistance pyrometry and in the comparison of nominally equal 

 resistances. 



A practical difficulty is that the exact adjustment of a sensitive 

 instrument is somewhat troublesome and when made is not per- 

 manent, being subject to changes in 

 the levelling of the galvanometer. 

 Therefore, methods have been sug- 

 gested where the deflection is not 

 brought exactly to zero. 1 



Kohlrausch Method of Using a 

 Differential Galvanomete r. 2 For 

 work of the highest class, such as the 

 precision comparison of resistance 



standards, the method employed must KohiMWMch metS^d f of 

 be free from errors due to variations using a differential galva- 

 in the resistances of the galvanometer 



circuits. These might be caused by changes of connections, 

 involving the alteration of contact resistances, or they might be 

 due to the inclusion of potential terminals of the resistances 

 during the test but not when making the preliminary adjustment 

 for differentiality. 



Kohlrausch's method of overlapping shunts fulfils the desired 

 conditions. It is designed for the comparison of nominally 

 equal resistances. The scheme of connections is shown in Fig. 

 85. In carrying out the test some means of adjusting either S 

 or X, as well as one of the galvanometer circuits, is required. 



It is also necessary to be able to interchange B and a (equiva- 

 lent to interchanging the galvanometer circuits A and C). This 



