COMPENSATION METHODS. 



417 



The arrangement first used by Poggendorff is represented in 

 Fig. 204. 



The circuit of a constant battery E contains two rheostates 

 R and r, the latter being between the points A and B. The 

 couple ^, to be measured, is connected with the points A and B 

 through a galvanometer G, the currents produced by the electro- 

 motive forces E and e being all directed towards the point A. The 

 two rheostates are connected, so that there is no current in the 



Fig. 204. 



galvanometer. If, then, R is the resistance of the portion AEB of 

 the circuit, and r that of the bridge AB, 



If we wish to dispense with a determination of the resistances 

 R and r, they may be replaced by other values R' and r\ so as to 

 obtain again no current. We have then 



r' -r 



R'-R' 



and the ratio of the electromotive forces is given by the changes of 

 resistance in each of the two rheostates. 



1012. We may suppress the rheostate r, if we take for AB a 

 homogeneous wire (Fig. 205), and seek the point C of the wire with 

 which the second pole of the couple e must be connected, so that 

 there is no deflection of the needle.* If / is the total resistance of 

 the wire AB, x and x' the resistances of the lengths AC which satisfy 



* DUBOIS-REYMOND. Abh. der Berl Akad., p. 787. 1862. Ges. Abh., I, 

 p. 176. 



VOL. II. EE 



