A difference of potential in a circuit, or in part of a circuit, which
difference produces or is capable of producing a current, or is due to
the flow of such current.

It may be expressed as the fall in potential or the electro-motive force
included between any two points on a circuit. The current in an active
circuit is due to the total electro-motive force in the circuit. This is
distributed through the circuit in proportion to the resistance of its
parts. Owing to the distribution of electro-motive force throughout a
circuit including the generator, the terminals of a generator on closed
circuit may show a difference of potential far lower than the
electro-motive force of the generator on closed circuit. Hence potential
difference in such a case has been termed available electro-motive
force.