Direct-Current Voltmeters (D'Arsonval Type)

Author: E.E. Kimberly

As explained on page 128, a conductor carrying a current and having a direction perpendicular to a magnetic field is acted upon by a force which is proportional to the current. In a d-c voltmeter, the essential parts of which are indicated in Figs. 16-1 and 16-2, a constant field is produced by a permanent magnet.

Fig. 16-1. Essential Parts of Permanent-Magnet Moving-Coil Instrument

Fig. 16-2. Direct-Current Voltmeter

The very small current taken by the instrument flows through a coil of a few turns of wire mounted in the constant field in such manner that the force causes the coil to tend to rotate against the torque of two restraining spiral springs. Electrical connections to the coil are made through the two springs. A resistor in series with the coil limits the current to a few milliamperes. The current is proportional to the voltage applied to the instrument, and hence a scale swept by a pointer attached to the pivoted coil may be calibrated in volts. The voltage range to which a voltmeter is adapted is determined by the resistance of the series resistor. A Voltmeter is connected in a circuit in the manner indicated in Fig. 16-3.

Fig. 16-3. Direct-Current Voltmeter Connection