The electricity of friction belongs to the former category, and
the electricity of chemistry, heat, and magnetism to the latter.
The spark of a factional or influence machine can be compared to a
highland cataract of lofty height but small volume, which is more
picturesque than useful, and the current from a voltaic battery, a
thermopile, or a dynamo to a lowland river which can be dammed to
turn a mill. It is the difference between a skittish gelding and a
tame carthorse.

Not the spark from an induction coil or Leyden jar, but a strong
and steady current at a low pressure, is adapted for electrolysis
or electrodeposition, and hence the voltaic battery or a special
form of dynamo is usually employed in this work. A flash of
lightning is the very symbol of terrific power, and yet, according
to the illustrious Faraday, it contains a smaller amount of
electricity than the feeble current required to decompose a single
drop of rain.

In our simile of the mill dam and the battery or dynamo, the dam
corresponds to the positive pole and the river or sea below the
mill to the negative pole. The mill-race will stand for the wire
joining the poles, that is to say, the external circuit, and the
mill-wheel for the work to be done in the circuit, whether it be a
chemical for decomposition, a telegraph instrument, an electric
lamp, or any other appliance. As the current in the race depends
on the "head of water," or difference of level between the dam and
the sea as well as on the resistance of the channel, so the
current in the circuit depends on the "electromotive force," or
difference of potential between the positive and negative poles,
as well as on the resistance of the circuit. The relation between