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The Couloumb represents the quantity of electricity flowing in a circuit-where the rate of flow is one ampere per second.

In order to properly indicate comparative amounts of energy the element of time must also be taken into consideration. One watt passing for one hour is a watt-hour. Seven hundred and forty-six watts passing for one hour or one watt passing for seven hundred and forty-six hours is a horse-power hour.

The instruments used for measuring the amperage and voltage of a circuit are called respectively the ammeter and the voltmeter. That used for registering watt-hours is called the integrating watt-meter.

FIG. 12. Diagram Showing Alternating and Direct Current.

Direct current is current that passes or flows in one direction only. The current of all primary and secondary cells and of certain forms of dynamos is direct.

Alternating current is current that repeatedly reverses its direction of flow. A direct current may be represented by a straight line. An alternating current is shown by a wavy line crossing and recrossing a straight line. The current gradually rises from zero to a maximum and then dies away. It does not stop at this point however, but starts to rise again, this time flowing in a reverse direction. After reaching a maximum it dies away again and the cycle is repeated. From a to c represents a cycle and from a to b an alternation. Alternating currents usually have a frequency of 30, 60 or 120 cycles per second. Sixty is the most common frequency. Many wireless telegraph stations now employ currents having a frequency of 500 cycles.