[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.
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.