550 A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 



In determining T by experiment, it is best to make the alteration of resist- 

 ance in one of the arms by means of the arrangement described by Mr Jenkin 

 in the Report of the British Association for 1863, by which any value of p 

 from 1 to 1*01 can be accurately measured. 



We observe (a) the greatest deflection due to the impulse of induction 

 when the galvanometer is in circuit, when the connexions are made, and when 

 the resistances are so adjusted as to give no permanent current. 



We then observe (/J) the greatest deflection produced by the permanent 

 current when the resistance of one of the arms is increased in the ratio of 

 1 to p, the galvanometer not being in circuit till a little while after the con- 

 nexion is made with the battery. 



In order to eliminate the effects of resistance of the air, it is best to vary 

 p till )8 = 2a nearly ; then 



(28). 



If all the arms of the balance except P consist of resistance coils of very 

 fine wire of no great length and doubled before being coiled, the induction 

 coefficients belonging to these coils will be insensible, and T will be reduced 



to lp. The electric balance therefore affords the means of measuring the self- 

 induction of any circuit whose resistance is known. 



(46) It may also be used to determine the coefficient of induction between 

 two circuits, as for instance, that between P and S which we have called ra; 

 but it would be more convenient to measure this by directly measuring the 

 current, as in (37), without using the balance. We may also ascertain the 



fin rt 



equality of *p and i< by there being no current of induction, and thus, when 



we know the value of p, we may determine that of q by a more perfect method 

 than the comparison of deflections. 



Exploration of the Electromagnetic Field. 



(47) Let us now suppose the primary circuit A to be of invariable form, 

 and let us explore the electromagnetic field by means of the secondary circuit 

 ft, which we shall suppose to be variable in form and position. 



