The pressure and quantity of electricity given off continuously by open
and closed circuit batteries is very different.
The pressure (or "electromotive force") of one cell of an ordinary
open-circuit battery is only about one volt, and the current is usually
very much less than one ampère, except in a dry cell, which may give
more.
In the closed-circuit batteries described, the electromotive force
of each cell is about two volts, while the current varies from 1 to
perhaps 50 ampères, according to the size of the zinc and carbon plates.
It would not matter if you made one cell as big as a barrel, nor if you
put in a _dozen carbons and zincs_, the _electromotive force would not
exceed the volts mentioned for each type of battery_, but the _ampère
capacity would be greater_ than in a smaller cell on account of the
larger size of the carbon and zinc plates.
_Internal Resistance._--There is one other point which affects the
number of ampères which can be obtained from a closed-circuit battery,
and that is whether there is a large or small internal resistance in
the battery itself.
This depends upon the solution which is used and the arrangement of the
plates.
If there is a high resistance in the battery itself (called "internal
resistance"), the electricity must do work to overcome this resistance
before it can get out of the battery to do useful work through the
wires, and, consequently, the capacity in ampères is limited.
If, on the other hand, there is very little resistance in the battery,
the current has very little work to flow to the wires leading from the
battery, and we can get a larger quantity, or greater number of ampères.
Thus, you will see that while the closed-circuit battery is the
stronger, and will do all that the open-circuit battery will do, and
even more, in a short time the latter, though weaker, will do about as
much work for the same amount of zinc and carbon as the former, but
takes a much longer time.