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. [Illustration: Fig. 58] Now let us return to the volt meter itself. By referring to Fig. 55, we see that it requires .024 ampere to move the needle of the volt meter clear across the scale, and we have found that one fluid cell was able to send enough current through the resistance of the armature to move the needle two thirds of the way across the scale. At this point we find Fig. 1, which might be read "one-cell pressure." We prefer to commemorate the name of one of the workers in the field of electricity and call this pressure a "volt" after Alessandro Volta (1745-1827), born at Como, Italy. It is the electric pressure which is produced by one fluid cell of a certain kind. We say, then, that one volt pushes through the resistance of this armature .016 ampere. Half a volt would push through the resistance of the armature half as much current or .008 ampere. At this point we put .5. Thus each of the figures in the lower row (Fig. 55) shows what part of a volt is required to send enough current through this particular armature to move the needle to that point. [Illustration: Fig. 59]