A switch is connected to the battery so that the voltage of a varying number of cells may be passed through the meters. In order to secure a close adjustment of the voltage, a rheostat is placed across the battery and switch so that it shunts the cells which are in circuit. By adjusting both the rheostat and the switch, any voltage within the maximum range of the battery may be secured. After the meters have been calibrated and assembled, they are ready for service and will form a very useful and valuable part of the experimenter’s laboratory. When using the meters remember that it is always necessary that the ammeter shall be in series and the voltmeter in parallel or shunt with the circuit. CHAPTER VII. CURRENT CONTROL DEVICES. How to Make a Pole Changing Switch or Current Reverses How to Reverse a Small Motor. The switch shown in Figure 76 is a very simple device which will be found very handy for many purposes. It consists of two brass levers mounted alongside of each other and connected together with an insulating strip or yoke bearing a handle so that they can both be moved together. The levers are pivoted at their back ends and provided with binding posts marked B. P. and B. P. in the illustration. The front ends of the levers swing over three switch contact points properly spaced so that the levers touch the adjoining contacts at the same time. The contacts are marked 1, 2, and 3. [Illustration: FIG. 76.—A Pole changing Switch for reversing Small Motors or the direction of an Electric Current.] We will suppose that the experimenter has some device operated by a battery and desires to arrange the apparatus so that he can quickly reverse the terminals of the battery so as to send the current through the device in either direction quickly and at will. The apparatus to be operated should be connected to the two binding posts mounted on the switch levers and marked B. P. and B. P. Connect the negative pole of the battery to the contact marked 2 (the center one), and the positive pole to both 1 and 3 (the outside ones). When the switch is thrown to the right so that the levers rest on the contacts 2 and 3, the right hand lever will be positive and the left hand one negative. When the switch is thrown to the left so that the levers rest on 1 and 2, the left hand lever will be positive and the right hand one negative. The current has been reversed. [Illustration: FIG. 77.—Top view of a small Battery Rheostat]