In the manually operated digital megohmmeter, the electromagnetic induction effect is used to generate the test voltage, that is, the armature is arranged to move in the magnetic field, and the electronic megohmmeter generates the test voltage. As the voltage in the external circuit increases, the deflection of the indicator increases. The deflection of the indicator decreases as the current increases. Therefore, the resultant torque is proportional to the voltage and inversely proportional to the current.

When the circuit under test is disconnected, the torque caused by the voltage coil will reach a maximum value. And the pointer shows "infinity" to indicate that the entire circuit is not short-circuited. And the resistance in the circuit under test becomes larger. If there is a short circuit, the pointer displays "zero", indicating the "no" resistance in the circuit under test.

Based on the working principle of an ohmmeter or a ratio meter. Due to the magnetic field generated by voltage and current, the megohmmeter tester will generate a deflection torque, similar to "Ohm's Law".

The torque of the megohmmeter changes with the change of V/I. The resistance to be measured is connected to both ends of the generator and is connected in series with the deflection coil.

If current is supplied to the coil, the torque generated should be in the opposite direction.

1. High resistance = no current: If the resistance is very high (ie the infinity position of the pointer), no current is allowed to flow through the deflection yoke.

2. Small resistance = large current: If the circuit measures a small resistance, a large current is allowed to pass through the deflection yoke, that is, the generated torque makes the pointer point to "zero".

3. Intermediate resistance = variable current: If the measured resistance is intermediate, the torque generated will be aligned or the pointer will be set between "zero to initial" range.