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.

The deposition of a metal from a solution of its salt is very
easily shown in the case of copper. In fact, we have already seen