One of the first units usually required, in order to make intelligent comparisons, is a unit of measure. The _quart_ is the unit of _measure_ commonly applied to liquids and is based upon the amount of space occupied by a certain volume. The _pound_ is a unit of weight which determines a certain amount of any substance by comparing the force which gravity exerts in pulling it to the earth with the same effect of gravity on another standard "weight." Electric current is invisible and weightless, and for these and other reasons cannot be measured by the quart or weighed by the pound. The only way that it can be measured is by means of some of the effects which it produces. Either the chemical, electro-magnetic, or the heating effects may be made the basis of a system of measurement. 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.