144 REPORT—1863. 
duce a permanent deflection. 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 electromag- 
netic 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. Multi- 
plying the dimensions of C by T, we find the dimensions of Q are L? M?. 
26. Electric Capacity of a Conductor —It is found by experiment that, 
other circumstances remaining the same, the charge on an insulated conductor 
is simply proportional to the electromotive force between it and the surround- 
ing conductors, or, in other words, to the difference of potentials (47). The 
charge that would be produced by the unit electromotive force is said to 
measure the electric capacity of a conductor. Thus, generally, the capacity 
of a conductor S= where Q is the whole quantity in the charge produced 
by the electromotive force, E. When the electromotive force producing the 
charge is capable of maintaining a current, the capacity of the conductor may 
be obtained without a knowledge of the value either of Q or E, provided we 
have the means of measuring the resistance of a circuit in electromagnetic 
measure. For let R be the resistance of a circuit, in which the given elec- 
tromotive force, E, will produce the unit deflection on a tangent galvanometer, 
then, from equations (6) and (12), we have 
tsin 47 
S=2 = e . . . . . . . 14 
; wh, z see 
where ¢ and 7 retain the same signification as in equation (13) ($ 25). 
27. Direct Measurement of Electromotive Force.—The meaning of the words 
“ electromotive force” has already been explained ($16); this force tends to 
do work by means of a current or transfer of electricity, and may therefore 
be said to produce and maintain the current. In any given combination in 
which electric currents flow, the immediate source of the power by which the 
work is done is said to produce the electromotive force. The sources of power 
producing electromotive force are various. Of these, chemical action in the 
voltaic battery, unequal distribution of temperature in circuits of different 
conductors, the friction of different substances, magnetoelectric induction, 
and simple electric induction are the most familiar. An electromotive force 
may exist between two points of a conductor, or between two points of an 
insulator, or between an insulator and a conductor,—in fine, between any 
points whatever. This electromotive force may be capable of maintaining a 
current for a long time, as in a voltaic battery, or may instantly cease after 
producing a current of no sensible duration, as when two points of the atmo- 
sphere at different potentials (§ 47) are jomed by a conductor; but in every 
case in which a constant electromotive force, E, is maintained between any two 
points, however situated, the work spent or gained in transferring a quantity, 
Q, of electricity from one of those points to the other will be constant; nor 
will this work be affected by the manner or method of the transfer. If the 
electricity be slowly conveyed as a static charge on an insulated ball, the work 
-will be spent or gained in accelerating or retarding the ball; if the electricity 
be conveyed rapidly through a conductor of small resistance, or more slowly 
through a conductor of great resistance, the work may be spent in heating 
the conductor, or it may electrolyze a solution, or be thermoelectrially or 
mechanically used; but in all cases the change effected, measured as equiva- 
