ABSOLUTE ELECTROMAGNETIC MEASUREMENTS. 



419 



joined by galvanometers to two different points c and c' of the wire 

 AB, chosen so that simultaneously there is no current in the two 

 galvanometers ; this is the arrangement known as Clark's potentiometer. 



Between two points A and B (Fig. 206), that one of the two 

 couples which has the greatest electromotive force, e' for instance, is 

 inserted along with a galvanometer G', and the rheostate is regulated 

 so that the needle is at zero. The second couple e is connected 

 on the one hand to the point A, and on the other to the movable 

 contact C, which is then displaced until there is no current in the 

 corresponding galvanometer G. This galvanometer may be sup- 

 pressed if necessary, for after the first adjustment the needle can 

 only remain at zero provided no current passes in A^C ; but the 

 sensitiveness is then far less. 



In all these methods, compensation is not effected at the first 

 trial, and during the subsequent trials to get at the position of 



Fig. 206. 



equilibrium, the couple whose electromotive force is to be measured 

 produces currents, and is therefore more or less polarized. This 

 objection is partially obviated by inserting in the circuit a key, 

 which is pressed down for a very short time. Nevertheless, with 

 elements which are easily polarized, it is better, before proceeding 

 to the final reading, either to recharge the element or to leave it 

 at rest so long that the polarization disappears. 



1014. ABSOLUTE ELECTROMAGNETIC MEASUREMENTS. The 

 method of compensation gives, in absolute value, a measure of 

 the electromotive force, if we know the resistance r between the 

 points of the shunt and the -intensity I of the current which 

 traverses this resistance. This is the method used by Latimer 

 Clark* in measuring the electromotive force of his standard 

 element. 



* Phil. Trans. Roy. Soc., 1873. Journal of the Society of Telegraph 

 Engineers, VOL. vii., p. 85. 



EE2 



