The voltmeter is an instrument used to determine the voltage of a circuit. It consists of a light, rectangular coil of copper wire wound upon an aluminum frame, pivoted in jeweled bearings, and capable of rotating in a space between a soft iron core and the pole of a permanent magnet. A light tubular pointer, attached to the coil. moves over a graduated scale. The current is introduced into the coil by means of two spiral springs which serve to control the movement of the pointer. When a current passes through the wire, the coil tends to turn in a certain direction against the action of the springs which tend to hold it in place. The amount of deflection is proportional to the voltage. The scale is graduated to read in volts. It is accurate and substantial. This instrument should not be placed in a strong field, as such a field will permanently affect the permanent magnet. In this case the scale must be graduated again.
Fig.79. - D'Arsonsal Galvanometer.
Fig. 80. - Ammeter.