Electricity is measured by its effects, or the work it does; the greater the effect, or the work done, the greater is the quantity of electricity.
If we have a closed circuit which has a resistance of one ohm and we apply to it an e.m.f. (pressure) of 1 volt, then 1 ampere will flow through the circuit. The coulomb is the commonly used unit of quantity or amount of electricity, just as the gallon is the commonly used unit of quantity or amount of liquids.
A coulomb of electricity, so it has been estimated, is about 16 million million million electrons, free and ready to move. Therefore an ampere is this amount of electrons moving in a wire past a definite section in one second.
In other words, the term ampere has been applied to a rate of flow of a coulomb of electricity per second. Therefore as current of one ampere is a current flowing at the rate of a coulomb per second. It so happens in practical work we nearly always are interested in the rate of flow of electricity in amperes and seldom in the amount of electricity, coulombs. If an ampere flows in a circuit for 1 hour, how much electricity has passed? Q, the symbol for quantity of electricity, is 1 x 60 x 60 or 3,600 coulombs. This is referred to by Radio-Tricians as an ampere-hour, meaning that 3,600 coulombs, or an ampere has flowed for 1 hour.
CONNECTING CELLS AND BATTERIES
Dry cells are limited in the amount of current they will deliver. The ordinary dry cell is capable of producing with its e.m.f. 25 amperes of current in an external circuit.
If a dry cell could deliver 25 amperes for 4 hours, its electrical capacity would be 25 x 4 or 100 ampere-hours. But suppose we want twice that capacity--we could arrange two dry cells as shown in Figure 10(D). If we wanted 3 times the capacity of 1 cell, we would arrange 3 cells as in (F) or as in (H) for 4 times the capacity. Four times the capacity of 1 cell would be 25 x 4 x 4 which equals 400 ampere-hours. These are called a parallel arrangement (a parallel battery) and although we are increasing the capacity by adding more cells, the e.m.f. produced will always be the same as that of a single cell--1½ volts.
For many kinds of work 1½ volts is not enough. If we connect three dry cells in the manner shown in Fig. 10(A)—that
