For the purpose of measuring resistances up to a few thousand ohms, the most convenient appliance is a Wheatstone's Bridge (q.v), but when the resistance of the conductor to be measured is several hundred thousand ohms, or if it is the resistance of a so-called insulator, such as the insulating covering of the copper wires employed for distributing electric current in houses and buildings for electric lighting, then the ohmmeter is more convenient.
An ohmmeter in one form consists of two pairs of coils, one pair called the series coil and the other called the shunt coil.
Suppose it is desired to measure the insulation-resistance of a system of electric house wiring; the ohmmeter circuits are then joined up as shown in fig.
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.
In making the test, the whole of the copper wires belonging to any section of the wiring and the test must be connected together at some point and then connected through the series coil of the ohmmeter with one terminal of the dynamo.
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.
In the Nalder ohmmeter the electrostatic principle is employed.
In this case the dynamo and ohmmeter are combined in one instrument.
OHMMETER, an electrical instrument employed for measuring insulation-resistance or other high electrical resistances.