It should be understood that an electric dynamo or battery does not
generate electricity, for if it were only the quantity of electricity
that is desired, there would be no use for machines, as the earth may
be regarded as a vast reservoir of electricity, of infinite quantity.
But electricity in quantity without pressure is useless, as in the
case of air or water, we can get no power without pressure, a flow of
current.
As much air or water must flow into the pump or blower at one end, as
flows out at the other. So it is with the dynamo; for proof that the
current is not generated in the machine, we can measure the current
flowing out through one wire, and in through the other—it will be found
to be precisely the same. As in mechanics a pressure is necessary to
produce a current of air, so in electrical phenomena an electro-motive
force is necessary to produce a current of electricity. A current in
either case can not exist without a pressure to produce it.