The following figures, 48, 49 and 50, are three views of the instrument as it appears when looking down upon it, in its three changes. First, that in which the current is broken and the needle vertical. Second, in which the circuit is closed and the needle deflected to the right. Third, in which the circuit is closed and the needle deflected to the left. Each figure has, in connection with the pole changer, the battery, or any other generator of the electric fluid, represented by N and P, and the galvanometer represented by G. In each of the figures, the circles numbered 1, 2, 3, 4, 5, 6, 7, and 8, represent cups, filled with mercury, let into the wood of the platform, and made permanent. The small parallel lines terminating in these cups, represent copper wires or conductors. [Illustration: FIG. 48.] A, figure 48, represents a horizontal lever of wood, or some insulating substance, with its axis supported by two standards, B and C, by which it can easily vibrate. D represents an ivory ball, mounted upon a rod, inserted in the lever, and extending a few inches above it. It serves as a handle, by which to direct the elevation or depression of either end of the lever. Both ends of the lever branch out, presenting two arms each. Through each arm passes a copper wire, insulated from each other. The left hand branches support the wires which connect the mercury cups, 1 and 4, and 2 and 3, together. The right hand branches support the wires which connect the cups 5 and 7, and 6 and 8, together. The ends of these wires directly over the mercury cups are bent down, so that they may freely enter their respective vessels when required. The other wires are permanently secured to the platform. The position of the lever is now _horizontal_, and the bent ends of the wires, which it carries, are so adjusted, that none of them touch the mercury, consequently, there is no connection formed between the battery and galvanometer, and the needle is _vertical_. The ivory ball, it will be observed, is directly over the centre of the axis, and in that position required to break the circuit. Thus, the wires, 2 and 3, 1 and 4, 5 and 7, 6 and 8, are each out of the mercury, and the circuit being broken the fluid cannot pass. [Illustration: FIG. 49.]