Charge.  A conducting body insulated from all other bodies will receive and hold a certain amount of electricity (a charge), if subjected to an electrical potential.  Thus, referring to Fig. 119, if a metal plate, insulated from other bodies, be connected with, say, the positive pole of a battery, the negative pole of which is grounded, a current will flow into the plate until the plate is raised to the same potential as that of the battery pole to which it is connected.  The amount of electricity that will flow 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 closer together, it will be found that more current will now flow into each of them, although the difference of potential between the two plates must obviously remain the same, since each of them is still connected to the battery.