Q2Electronic Measurement & Instrumentation

There are two types: Attraction and Repulsion. In the Repulsion type, two soft iron vanes (one fixed, one attached to the moving pointer shaft) are placed inside a stationary coil. When current flows through the coil, it creates a magnetic field. Both iron vanes become magnetized with the same polarity. Since like poles repel, the moving vane is pushed away, turning the pointer against a control spring.

The deflecting torque equation is $T_d=\frac{1}{2}{I}^2\frac{dL}{d\theta }$.

Notice that the deflecting torque is proportional to the square of the current ($I^2$). - When measuring DC, $I^2$ is positive, providing a steady forward torque. - When measuring AC, the current rapidly reverses direction. However, during the negative half-cycle, $(-I{)}^2$ is still positive. Physically, both iron vanes reverse their polarity simultaneously, so they continue to repel each other in the exact same direction. The mechanical inertia of the pointer averages out the rapid torque pulses, displaying the RMS value of the AC current.