J. C. Maxwell’s, ‘Dynamical theory of the electromagnetic field’

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                                p/P - q/Q - r/R + s/S + h(1/P - 1/Q) + k(1/R - 1/P) + l(1/R + 1/Q) - m(1/P + 1/S) + n(1/Q - 1/S) + o(1/S - 1/R) = [tau] (26) 
we find Z/z = 2 sin[half][alpha]/tan[theta] T/[pi] = [tu]/1 - [rho] (27)
In determining [tau] 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 [rho] from 1 to 1.01 can be accurately 

<s>If<\s> We observe [alpha], the greatest deflexion 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 [beta] the greatest deflexion produced by the 
permanent current when the resistance of one of the arms is 
increased in the ratio of 1 to [rho], 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 [rho] till [beta] = 2[alpha] nearly then 
[tau]= T 1/[pi](1 - [rho]) 2 sin[half][alpha] / tan[half][beta] (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 [tau] will be reduced to 
p/P. 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, 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 p/P and q/Q by there being no 

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James Clerk Maxwell
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Cite as

J. C. Maxwell’s, ‘Dynamical theory of the electromagnetic field’, 1864. From The Royal Society, PT/72/7



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