21. _Arc Lamp Feed._--As will be explained later, an arc lamp must have its carbons touching one another when the current is first thrown on, and then the carbons must be drawn apart from a quarter to half an inch. The upper carbon is lifted away from the lower one by a portion of the current passing by means of a shunt around an electro-magnet. [Illustration: Fig. 50] [Illustration: Fig. 51] [Illustration: Fig. 52] 22. _Volt meter._--The volt meter measures the pressure of an electric current. The volt meter which we examined looked outside like our ammeter, and when we removed the face it appeared inside like an ammeter. There was the steel magnet of horseshoe shape to furnish a field (Fig. 51), and there was an electro-magnet poised between its poles for an armature. The armature in the volt meter, however, had wound upon it finer wire and more of it than was the case in the ammeter. There was no shunt wire in the volt meter as there was in the ammeter. We connected in series a fluid cell (to be described later), the ammeter, and the volt meter (Fig. 52). The ammeter shunt was removed so that all the current went through its armature. The volt meter needle went to one which was two thirds of the scale (Fig. 53), and the ammeter needle indicated .016. That is, this particular cell can push sixteen thousandths of an ampere through the resistance of this volt meter, and .016 ampere passing through the armature of this volt meter will magnetize it sufficiently to move it against its spring, say sixty degrees. [Illustration: Fig. 53] [Illustration: Fig. 54] [Illustration: Fig. 55] 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). [Illustration: Fig. 56]