Fig. 54
We put into the circuit a lot more fine wire for resistance, R ([Fig. 54]), so that the volt meter needle went only half as far as before, that is to .5. The ammeter indicated only half as much as before, that is .008 ampere. We put in resistance enough to bring the volt meter needle down to .25 and the ammeter indicated one quarter of the original current. We put in less resistance, bringing the volt meter needle to .75, and the ammeter indicated three fourths of the original current. Evidently the volt meter is merely an ammeter with a different scale marked upon its card. With a pen we marked upon the card of the volt meter a true ammeter scale ([Fig. 55]).
Fig. 55
Fig. 56
In order to understand the volt meter, let us turn our attention for a moment to [Fig. 56]. I have arranged the water tank T at such a height above the faucet F that when the faucet is opened one quart of water will flow in a minute. If I partially close the faucet, making the opening one half as large (that is, offering twice the resistance to the flow), half a quart will flow in a minute. If I make the resistance four times as great only one quarter of a quart will flow in a minute. It is evident that I could arrange a scale underneath the handle of the faucet to indicate the quantity of water flowing, just as the ammeter and volt meter indicate the quantity of electricity which flows. If now that much is understood, it will be easy to learn how the water faucet may be used to measure water pressure and the volt meter in like manner used to measure electric pressure.