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 these is expressed by the well-known law of Ohm, which runs:  A current of electricity is directly proportional to the electromotive force and inversely proportional to the resistance of the circuit.

In practice electricity is measured by various units or standards named after celebrated electricians.  Thus the unit of quantity is the coulomb, the unit of current or quantity flowing per second is the ampere, the unit of electromotive force is the volt, and the unit of resistance is the ohm.

The quantity of water or any other “electrolyte” decomposed by electricity is proportional to the strength of the current.  One ampere decomposes .00009324 gramme of water per second, liberating .000010384 gramme of hydrogen and .00008286 gramme of oxygen.

The quantity in grammes of any other chemical element or ion which is liberated from an electrolyte or body capable of electrochemical decomposition in a second by a current of one ampere is given by what is called the electrochemical equivalent of the ion.  This is found by multiplying its ordinary chemical equivalent or combining weight by .000010384, which is the electrochemical equivalent of hydrogen.  Thus the weight of metal deposited from a solution of any of its salts by a current of so many amperes in so many seconds is equal to the number of amperes multiplied by the number of seconds, and by the electrochemical equivalent of the metal.