CONSTANTS OF COILS. 



The surface of a coil is nothing but the coefficient of mutual 

 induction of this coil with a circuit capable of giving with unit 

 current, a uniform field equal to unity. The methods pointed out 

 previously for comparing surfaces may then be used for the co- 

 efficients of mutual induction. 



Let us consider, for instance, two systems of coils A and a, 

 A' and #', the coefficients of mutual induction of which are re- 

 spectively M and M', and let us suppose the two systems without 

 action on each other. The coils A and A' are placed in the circuit 

 of the same inducing current I, the coefficients M and M' are then 

 proportional to the flows of force Q and Q', from the inducing current 

 into the coils a and a respectively. If a ballistic galvanometer is 

 interposed in a circuit which contains the coils a and a\ the con- 

 nections are so arranged that the induced currents due to the flows 



of force Q and Q', are alternately in the same and opposite directions. 

 The ratio of the swings a and a' (1092) will give then the ratio 

 Q' M' 

 Q r M" 



In like manner, a differential galvanometer, the two frames of 

 which are in the circuits of the coils a and a' respectively, will give 

 the ratio of the coefficients M and M', by the ratio of the total 

 resistances of the two circuits, when the condition of equilibrium 

 is realised. 



These methods present special difficulties, and possess no great 

 accuracy. It is better to attempt to produce a compensation of 

 induced currents analogous to that which Wheatstone's bridge gives 

 for permanent currents. 



1095. In the method described by Maxwell,* the inducing 



* MAXWELL. Electricity and Magnetism, Vol. II., p. 355. 



