PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD. 
477 
Then calling the quantity 
p~Q~R+ii + K^ _ 6) + Ki~s) +o G~s) =*■ 
we find 
Z 2 sin T t 
2 tail 0 5T 1 — q 
(27) 
In determining r by experiment, it is best to make the alteration of resistance 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 g from 1 to T01 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 (/3) the greatest deflection produced by the permanent current when 
the resistance of one of the arms is increased in the ratio of 1 to g, the galvanometer 
not being in circuit till a little while after the connexion is made with the battery. 
In order to eliminate the effects of resistance of the air, it is best to vary g till j3 = 2a 
nearly; then . _ . , 
W 
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 r will be reduced to ^ . The electric balance there- 
fore 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 m ; 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 equality of ^ and F 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 B, which we shall 
suppose to be variable in form and position. 
We may begin by supposing B to consist of a short straight conductor with its extre- 
mities sliding on two parallel conducting rails, which are put in connexion at some 
distance from the sliding-piece. 
3 t 2 
