A quantity of electricity, like a quantity of water, may be measured. Since the flow or quantity of water depends on the pressure or "head" and on the resistance of the pipes, so the quantity of electricity depends upon the pressure and the resistance of the wires.
The acting force which gives rise to, or maintains, a current or flow of electricity is called the electromotive force (abbreviated E.M.F.). The E.M.F. corresponds to pressure in relation to water and is measured by a unit called a volt. That force against which the E.M.F. acts, that is to say, that force which retards the flow or current, is called the resistance, and corresponds to the friction of pipes in relation to water. Resistance is measured by a unit called an ohm. The quantity of electricity corresponds to the quart or gallon of water. The current of electricity, or rate of flow, is measured by a unit called a coulomb, which is the quantity passing per second of time, and corresponds to a flow of water of so many quarts or gallons per second. A rate of flow of one coulomb per second is called an ampere. The unit of rate of electrical work is the product of the E.M.F. and the rate of flow or current - just as the pressure with which the force acts is the work performed. The rate of flow of electricity or current is proportional to the impelling pressure or head.