The term is employed to denote that which moves or tends to move
electricity from one place to another. For brevity it is written E. M.
F.; it is the result of the difference of potential, and proportional
to it. Just as in water pipes, a difference of level produces a
pressure, and the pressure produces a flow so soon as the tap is
turned on, so difference of potential produces electro-motive force,
_and electro-motive force sets up a current so soon as a circuit is
completed for the electricity to flow through_. Electro-motive force,
therefore, may often be conveniently expressed as a difference of
potential, and _vice versa_; but the reader must not forget this
distinction.
In ordinary acceptance among engineers and practical working
electricians, electro-motive force is considered as pressure and it
is measured in units called volts. The usual standard for testing and
comparison is a special form voltaic cell, called the Clark cell. This
is made with great care and composed of pure chemicals.
The term _positive_ expresses the condition of the point having the
higher electric energy or pressure, and, _negative_, the lower relative
condition of the other point, and the current is forced through the
circuit by the (E. M. F.) electric pressure at the generator, just as a
current of steam is impelled through pipes by the generating pressure
at the steam-boiler.
Care must be taken not to confuse electro-motive force with electric
force or electric energy, when matter is moved by a magnet, we speak
rightly of magnetic force; when electricity moves matter, we may speak
of electric force. But, E. M. F. is quite a different thing, not
“force” at all, for it acts not on matter but on electricity, and tends
to move it.