In the Nalder ohmmeter the electrostatic principle is employed.
Connect the ohmmeter across the contacts of the switch and actuate the switch's arm several times while checking the meter reading.
An ohmmeter in one form consists of two pairs of coils, one pair called the series coil and the other called the shunt coil.
For this purpose the ohmmeter is provided with a small dynamo D, contained in a box, which produces a continuous electromotive force of from 200 to 500 volts when the handle of the instrument is steadily turned.
On setting the dynamo in operation, a current passes through the shunt coil of the ohmmeter proportional to the voltage of the dynamo, and, if there is any sensible leakage through the insulator to earth, at the same time another current passes through the series coil proportional to the conductivity of the insulation of the wiring under the electromotive force used.