The first method used to measure electric current was the chemical one.
If a current is passed through a solution of a chemical called copper sulphate (blue vitriol) by means of two copper plates, copper will be deposited on one plate and dissolved from the other. If the current is furnished by a battery the copper will be deposited on the plate connected with the zinc of the battery. If the current is allowed to flow for a short time and the two copper plates are then taken out and weighed it will be found that one plate is considerably heavier than the other.
The copper has been taken from one plate and deposited on the other by the electric currents. The amount of electric current which will deposit 1.177 grammes of copper in an hour is called an ampere. The ampere is the unit of electrical current measurement, and implies quantity or amount.
The chemical method of measuring current was at one time put to practical service in the distribution of electric current for lighting and power. Many years ago the house meters, used to measure the current, consisted of a jar containing two copper plates. The current used in the house would cause copper to deposit on one plate, and by weighing the plate the power company could determine the amount of current used, and thereby the amount of the bill. The meters nowadays make use of the magnetic effects of the current instead of the chemical, as described later on.
The Volt
For purposes of explanation the electric current may be likened to a stream of water flowing through a pipe.
If you hold your thumb over the end of a water-pipe through which water is flowing it will push your thumb away because of the pressure which the water exerts.
Electric currents also exert a pressure, only it is not called pressure in electrical parlance, but, spoken of as electromotive force or potential.
The pressure of the water enables it to pass through small openings and to overcome the resistance offered by the pipe.
Wires and other electrical conductors do not offer a perfectly free path to an electric current, but also possess a resistance. It is the potential of the electro-motive force which overcomes the resistance and pushes the current through the wire.