developing light and heat as in the electric spark, chemical
action as m the voltameter, and magnetism as in the electromagnet.

The words "current," "pressure," and so on point to a certain
analogy between electricity and water, which helps the imagination
to figure what can neither be seen nor handled, though it must not
be traced too far. 'Water, for example, runs by the force of
gravity from a place of higher to a place of lower level. The
pressure of the stream is greater the more the difference of level
or "head of water" The strength of the current or quantity of
water flowing per second is greater the higher the pressure, and
the less the resistance of its channel. The power of the water or
its rate of doing mechanical work is greater the higher the
pressure and the stronger the current. So, too, electricity flows
by the electromotive force from a place of higher to a place of
lower electric level or potential. The electric pressure is
greater the more the difference of potential or electromotive
force. The strength of the electric current or quantity of
electricity flowing per second is greater the higher the pressure
or electromotive force and the less the resistance of the circuit
The power of the electricity or its rate of doing work is greater
the higher the electromotive force and the stronger the current.

It follows that a small quantity of water or electricity at a high
pressure will give us the same amount of energy as a large
quantity at a low pressure, and our choice of one or the other
will depend on the purpose we have in view. As a rule, however, a
large current at a comparatively low or moderate pressure is found
the more convenient in practice.