The two binding posts connected to the terminals of the wire on the bobbin should be mounted on the base. A small, round-headed brass screw, long enough to pass all the way through the base will serve to level the instrument and bring the pointer exactly at zero. If a little brass strip is placed in the slot in the screw head and soldered there so as to form what is known as a "winged screw", the adjustment may be made with the fingers and without the aid of a screw driver. The scale is formed on a piece of stiff white cardboard directly under the pointer. The scale is supported by gluing it to two small wooden blocks. The various values are marked on the scale with a pen and ink. The glass front should therefore not be put in place until the instrument has been calibrated. To properly calibrate the meters, they must be compared with a suitable standard. The zero value on the meters is normally in the centre of the scale. When a current is passed through the bobbins, the armature tends to swing around at right angles. But since the armature is pivoted above its centre of mass, the centre of gravity is displaced when it swings and exerts a pull in opposition to that of the bobbin. The amount of swing will be greater as the current is correspondingly stronger. The pointer will swing either to the right or to the left, depending upon the direction in which the current passes through the coil of wire on the bobbin. [Illustration: FIG. 73.—Details of the Wooden Parts which form the Case.] The zero point on the two instruments shown in the accompanying illustration is at the extreme left of the scales. The pointers are bent to the left so that the current may be passed through the meters in only one direction and the scales will have a greater range of values. In order to calibrate the ammeter, place it in series with a standard ammeter having a calibrated scale which is known to be correct. A set of strong batteries and a rheostat should be included in the circuit. The arrangement is illustrated in Figure 74. The rheostat is adjusted so that various current readings are obtained. The positions of the pointer on the standard meter are carefully noted and corresponding graduations made on the scale of the other meter for each value. [Illustration: FIG. 74.—Showing how the Apparatus is arranged and connected for calibrating the Ammeter.] [Illustration: FIG. 75.—Showing how the Apparatus is arranged and connected for calibrating the Voltmeter.] In order to calibrate the voltmeter it must be compared to a standard voltmeter. The voltmeters must be placed in parallel or shunt with each other as shown in Figure 75.