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144 pole of the battery, and is known as the 'positive' (+). The outer zinc case forms the other or 'negative' pole (–).

Poles.—When coupling up a number of cells to form a battery, the carbon of one cell is connected to the zinc of the next, and so on, until all the cells are connected, leaving one free wire at each end of the battery. These wires are known as the positive and negative poles of the battery.

Pressure of Electricity ( Volts). Each cell is capable of giving forth a certain small pressure of electricity. Pressure of electricity may be compared to the pressure of water in a pipe, or steam in a boiler, and is measured in units of pressure (volts) which may be said to correspond to pounds per square inch.

Flow of Electricity (Amperes).—Before, however, any current of electricity can pass out of the cell a complete 'circuit' must be formed between the two 'poles.' A quantity of electricity will then pass round this circuit in proportion to the pressure (volts) in the cell. This quantity or flow is measured in units known as 'amperes.' One ampere flowing for one hour is known as an 'ampere-hour' and the capacity of a battery is measured in ampere-hours.

Coupling in Parallel.—If the capacity of one battery is insufficient, two or more may be joined up in parallel by connecting the positive poles together and the negative poles together. To obtain a sufficient spark a battery must be capable of giving out a pressure of at least four 'volts.' For this reason it is usual to couple up four dry cells together.

Disadvantage of Dry Cells.—As we have explained, the utility of a battery depends on its capacity in ampere-hours. Every dry battery has a rated capacity, and once this quantity of electricity has been drawn from it, it becomes practically worthless, and a new one is required. The cost of these dry cells is very considerable, and if they are left for some time without work it not infrequently happens that the capacity has been seriously impaired owing to electrical leakage, which is