156 



HENRY A. EOWLAND 



THEORY OF THE METHOD 



When a current is induced in a circuit by magnetic action of any kind, 

 Faraday has shown that the induced current is proportional to the 

 number of lines of force cut by the circuit and inversely as the resist- 

 ance of the circuit. If we have two circuits near each other, the first 

 of which carries a current, and the second is then removed to an infinite 

 distance, there will be a current in it proportional to the number of 

 lines of force cut. Let now a unit current be sent through the second 

 circuit and one of strength E through the first; then, on removing 

 the second circuit, work will be performed which we easily see is also 

 proportional to the number of lines of force cut. Hence, if EM is 

 the work done, Q is the induced current, and R is the resistance of the 

 second circuit, 



-, 



where C is a constant whose value is unity on the absolute system. 



When the current in the first circuit is broken, the lines of force 

 contract on themselves, and the induced current is the same as if the 

 second circuit had been removed to an infinite distance. If the current 

 is reversed the induced current is twice as great; hence in this case 



= ^ or = 



K V 



Hence, to measure the absolute resistance of a circuit on this method, 

 we must calculate M and measure the ratio of Q to E. M is known 

 as the mutual potential of the two circuits with unit currents, and 

 mathematical methods are known for its calculation. 



The simplest and best form in which the wire can be wound for the 



