292 MEASUREMENT OF CURRENTS. 



two branch wires. The amplitudes A, A', and A of the current 

 in the two branches and in the principal circuit satisfy the 

 equations 



A 2 A' 2 A; 



~ 



When the coefficient of self-induction of the wire, which acts as a 

 shunt, is zero, we have L = 0, M = 0, and therefore 



If the second term of the bracket is very small, which is the 

 ordinary case, we see that the use of shunts, with the precautions 

 mentioned, may also serve to determine alternate currents by the 

 aid of an electrodynamometer. 

 T/ 



If the ratio had any considerable value, we might eliminate 



r + r' 



it by two different experiments in which one of the resistances r or r' 

 is varied. 



905. MEASUREMENT OF ALTERNATING CURRENTS BY THE 

 ELECTROMETER. The most correct means of measuring alternating 

 currents is that which has been pointed out in (868), and which 

 consists in connecting, with two points A and B of the circuit, 

 the two electrodes of a quadrant electrometer the needle of which 

 is connected with one pair of quadrants.* The deflection of the 

 needle, being proportional to the square of the difference of po- 

 tential, does not change its sign with this difference. If then 

 currents, which are alternately in opposite directions, succeed each 

 other at very short intervals compared with the time of oscillation 

 of the needle, this, like the movable coil of the galvanometer, 

 takes up a fixed deflection proportional to the mean square of the 

 intensity. 



JOUBERT. Cotnptes rendus, Vol. xci., p. 161. 1880. 



