The number of watts is found by multiplying the voltage by the amperage. In the case of the sounder and battery used as an example to explain Ohm’s Law, and where the voltage was 10 and the amperage found to be 2, the number of watts is 10 x 2, or 20 watts. Seven hundred and forty-six watts represent one electrical horse-power. One thousand watts are called a _kilo-watt_. The Coulomb So far, none of the units have taken into consideration the element of time. If water should be permitted to run out of a pipe into a tank until ten gallons had passed it would not be possible to tell at what rate the water was flowing by knowing that ten gallons had passed unless it were also known how long the water had been flowing. Ten gallons per minute or ten gallons per hour would indicate the rate of flow. One ampere flowing for one second is the electrical unit of flow. This unit is called the _coulomb_. One ampere flowing for one hour is called an _ampere hour_. The number of ampere hours is found by multiplying the current in amperes by the time in hours. A battery may be said to have a capacity of 10 ampere hours. This means that it will deliver one ampere for 10 hours (1 ampere x 10 hours = 10 ampere hours) or 2 amperes for 5 hours (2 amperes x 5 hours = 10 ampere hours). The same element of time enters into consideration in connection with the watt. One watt flowing for one hour is a _watt hour_ and one kilowatt flowing for one hour is a _kilo-watt hour_. The Difference between Alternating and Direct Currents There are two distinct kinds of electric current supplied for lighting and power, one known as _direct_ current and the other as _alternating_. A _direct current_ is one which passes in one direction only. It may be represented by a straight line, as _A_ in Figure 88. An alternating current is one which reverses its direction and passes first one way and then the other. It may be represented by a curved line, shown in Figure 88. It starts at _zero_, and gradually grows stronger and stronger. Then it commences to die away until no current is flowing. At this point it reverses and commences to flow in the opposite direction, rising gradually and then dying away again. This is repeated a definite number of times per second; when the current rises from zero, reverses and returns to zero, it is said to pass through a _cycle_. [Illustration: Fig. 88.—Graphic Representation of a Direct and an Alternating Current.] The part of the curved line from _a_ to _b_ in Figure 88 represents the first part of the current, when it is rising. From _b_ to _c_ represents its fall. The point at which the curved line crosses the straight line is zero. At _c_ the current crosses the line and commences to flow in the opposite direction until it reaches _d_, at which point it dies away and again crosses the line to flow in its original direction and _repeat the cycle_.