low into the plate
will depend, other things being equal, on the potential of the source
from which it is charged; in fact, it is proportional to the potential
of the source from which it is charged. This amount of electricity is
a measure of the capacity of the plate, just as the amount of water
that a bath-tub will hold is a measure of the capacity of the
bath-tub.

Capacity. Instead of measuring the amount of electricity by the
quart or pound, as in the case of material things, the unit of
electrical quantity is the _coulomb_. The unit of capacity of an
insulated conductor is the _farad_, and a given insulated conductor is
said to have unit capacity, that is, the capacity of one farad, when
it will receive a charge of one coulomb of electricity at a potential
of one volt.

Referring to Fig. 119, the potential of the negative terminal of the
battery may be said to be zero, since it is connected to the earth. If
the battery shown be supposed to have exactly one volt potential, then
the plate would be said to have the capacity of one farad if one
coulomb of electricity flowed from the battery to the plate before the
plate was raised to the same potential as that of the positive pole,
that is, to a potential of one volt above the potential of the earth;
it being assumed that the plate was also at zero potential before the
connection was made. Another conception of this quantity may be had by
remembering that a coulomb is such a quantity of current as will
result from one ampere flowing one second.

The capacity of a conductor depends, among other things, on its area.
If the plate of Fig. 119 should be made twice as large in area, other
things remaining the same, it would have twice the capacity. But there
are other factors governing the capacity of a conductor. Consider the
diagram of Fig. 120, which is supposed to represent two such plates as
are shown in Fig. 119, placed opposite each other and connected
respectively with the positive and the negative poles of the battery.
When the connection between the plates and the battery is made, the
two plates become charged to a difference of potential equal to the
electromotive force of the battery. In order to obtain these charges,
assume that the plates were each at zero potential before the
connection was made; then current flows from the battery into the
plates until they each assume the potential of the corresponding
battery terminal. If the two plates be brought clos