When we were estimating the energy required to climb the stairs of an apartment house, we needed to take into account two factors, (1) our weight and (2) the time which we took in climbing them. The amount of coal burned, steam generated, electricity produced, to run our elevator depends upon two factors, (1) its weight and (2) its speed. That idea is fundamental. Let us get at it in still another way. Suppose we have a mill pond, (Fig. 17, _A_). We construct a penstock _p_ and install a water-wheel, _S_, to operate a mill. Our business increases and we install more machinery in our mill and must have more power to run it. We have two ways of getting it, (1) we may lengthen our wheel and enlarge our penstock so that a greater weight of water will fall upon the wheel, or (2) we may lengthen our penstock and move the wheel farther down so that the water will fall upon the wheel with greater velocity. It is just so with the electric current. Like water it is driven on in its course by pressure. The unit for electric pressure is called a volt. If we wish to drive the wattmeter or any other electric motor twice as fast as now, we may choose whether we shall do so by doubling the volts of pressure or by doubling the amperes of quantity. The electric pressure on our mains is about one-hundred and ten volts. We three together weigh 330 pounds. Our elevator brought us up stairs at the speed of 100 feet per minute. It requires one horse-power to raise 330 pounds 100 feet in a minute. The ammeter in the engine room showed that 7 amperes of electricity were sent through the motor of the elevator to bring us up. That is, seven amperes at 110-volt pressure give one horse-power. In the office building across the street where they use a 220-volt current 3-1/2 amperes are required to take us up stairs at the same speed. It is necessary that the same amount of coal be consumed to furnish the horse-power of energy whether we supply it by means of seven amperes at 110 volts or 3-1/2 amperes at 220 volts. You notice that the product is 770 in each case. The name given to this product is _watts_. More accurately 746 watts of electrical power are equivalent to one horse-power. The name of this unit commemorates the famous inventor of the steam engine, James Watt (1736-1819). His monument now overlooks the Clyde at his native town, Greenock, Scotland. To light a certain lamp, to heat a certain laundry iron, to furnish a certain amount of power for an electric motor, we must have a definite number of watts. We may choose whether we will have it at high or low voltage with correspondingly low or high number of amperes. [Illustration: Fig. 18]