An ohm meter is a measuring instrument for measuring the electrical resistance of a component or an electrical circuit. The unit of measure is the ohm, noted Ω.

Principle of operation

Two methods can be used to measure the value of resistance :

Measure a voltage with a current generator;
Measurement of a current with a voltage generator (or D.D.P).

Current generator

Current generator requires an IM intensity through the unknown resistance RX, we measure the voltage VM appearing at its terminals.

Such an arrangement does not measure with precision of resistance whose value exceeds a few kΩ as the current in the voltmeter

Voltmeter

The voltmeter is a device that can measure the voltage (or difference in electrical potential) between two points, the size of which is the unit of measurement is the volt (V).
The vast majority of today's measuring devices are built around a digital voltmeter, with the physical size to be measured converted to voltage using an appropriate sensor. This is the case with the digital multimeter, which, in additi......

is then more negligible (the internal resistance of the voltmeter

Voltmeter

The voltmeter is a device that can measure the voltage (or difference in electrical potential) between two points, the size of which is the unit of measurement is the volt (V).
The vast majority of today's measuring devices are built around a digital voltmeter, with the physical size to be measured converted to voltage using an appropriate sensor. This is the case with the digital multimeter, which, in additi......

being commonly equal to 10 MΩ). Mounting is therefore supplemented by a current generator auxiliary enslaved to the value of the voltage measured by the voltmeter

Voltmeter

The voltmeter is a device that can measure the voltage (or difference in electrical potential) between two points, the size of which is the unit of measurement is the volt (V).
The vast majority of today's measuring devices are built around a digital voltmeter, with the physical size to be measured converted to voltage using an appropriate sensor. This is the case with the digital multimeter, which, in additi......

, to deliver the current in the noted voltmeter

Voltmeter

The voltmeter is a device that can measure the voltage (or difference in electrical potential) between two points, the size of which is the unit of measurement is the volt (V).
The vast majority of today's measuring devices are built around a digital voltmeter, with the physical size to be measured converted to voltage using an appropriate sensor. This is the case with the digital multimeter, which, in additi......

IV
When the value of the resistor RX is less than 10 ohms, to avoid taking into account various connection resistances, to implement a particular realized montage in 4-wire ohmmeters.

Voltage generator

We use an ammeter to measure current I flowing through a resistance RX to which one applies a low V tension set.

This method is used in the analog ohmmeters with galvanometer to mobile framework.