454 Prof. Maxwell and Mr. F. Jenkin on the Elementary 



Leyden phial, the quantity which one coating loses, or which the 

 other gains, is the electromagnetic unit quantity*. The mea- 

 surement thus defined of the quantity in a given statical charge 

 can be made by observing the swing of a galvanometer-needle 

 produced by allowing the charge to pass through the coil of the 

 galvanometer in a time extremely short compared with that oc- 

 cupied by an oscillation of the needle. 



Let Q be the whole quantity of electricity in an instantaneous 

 current, then ~ 



Q = 2^sinK!. ..... (12) 



where C, = the strength of a current giving a unit deflection 

 (45° on a tangent or 90° on a sine galvanometer), t = half the 

 period or time of a complete oscillation of the needle of the gal- 

 vanometer under the influence of terrestrial magnetism alone, 

 and i = the angle to which the needle is observed to swing from 

 a position of rest, when the discharge takes place. C { is a con- 

 stant which need only be determined once for each instrument, 

 provided the horizontal force of the earth's magnetism remain 

 unchanged. In the case of the tangent galvanometer, the for- 

 mula for obtaining it has already been given. From equations 

 (9) and (12) we have for a tangent galvanometer 



Q=4-H^sin|i, ..... (13) 



where, as before, k = the radius of the coil, and n = the number 

 of turns made by the wire round the coil. 



The quantity in a given charge which can be continually repro- 

 duced under fixed conditions may be measured by allowing a 

 succession of discharges to pass at regular and very short inter- 

 vals through a galvanometer, so as to produce a permanent de- 

 flection. The value of a current producing this deflection can 

 be ascertained ; and the quotient of this value by the number of 

 discharges taking place in the " second " gives the value of each 

 charge in electromagnetic measure. 



To find the dimensions of Q, we simply observe that the unit 

 of electricity is that which is transferred by the unit current in 

 the unit of time. Multiplying the dimensions of C by T, we 

 find the dimensions of Q are L* M T . 



26. Electric Capacity of a Conductor. — It is found by experi- 

 ment that, other circumstances remaining the same, the charge 

 on an insulated conductor is simply proportional to the electro- 

 motive force between it and the surrounding conductors, or, in 

 other words, to the difference of potentials (47). The charge 



* Weber calls this quantity two units — a fact which must not be lost 

 sight of in comparing his results with those of the Committee. 



