Having set the faucet so that a quart will flow per minute, let us put on a longer tube p, and move the tank up to another shelf so that the distance from the water level in the tank to the faucet is twice as great as before. Under the increased pressure water runs through the faucet twice as fast and we now get two quarts per minute.
I purposely placed the tank out of sight behind a partition so that you might practise judging the water pressure by the flow at the faucet. We cannot very well talk about pressure in quarts. We might talk about it in pounds, but if we used this apparatus much we should probably get into the habit of talking about the pressure from one shelf, two shelves, three shelves, etc.
In order that the pressure might remain nearly constant during the experiment we would probably introduce resistance (that is, partially close the faucet) so that the water level should not fall much. We might, for example, set the faucet so that half a pint would flow in a minute when the tank was on the first shelf. Then a pint per minute would flow when the tank was on the second shelf and one and a half pints per minute when the tank was on the third shelf, etc. Thus we should infer the pressure by measuring the quantity.
One more illustration and the case will be clear. To save the trouble of measuring the quantity of water which flows through the faucet, suppose I introduce the device represented in [Fig. 57]. W is a small water wheel comparable to the armature of the volt meter. It carries a pointer which moves over a scale just as in the case of the volt meter.
Fig. 57
It has a spring coiled around its axle which tends to keep the pointer at 0, as in the case of the volt meter. The tank is placed upon the first shelf, the faucet is fixed so that a small amount of water flows and the needle moves to a certain figure upon the scale. We will mark this point one and call it "first-shelf pressure." The tank is lifted to the second shelf and the index moves to another point, which we will mark two and call it "second-shelf pressure." The tank is lifted to the third shelf and the index moves to a third point, which we will mark three and call it "third-shelf pressure," etc.
Ordinarily we measure water pressure with an instrument which allows no water to run to waste, but in measuring electric pressure by the volt meter some current must pass through the instrument, just as in the case of our water-wheel illustration in [Fig. 57]. We put in large resistance so as to make this current as small as possible, while we let enough pass to move the armature.