Ohmmeter

A ohmmeter is a Measuring instrument which makes it possible to measure the Electrical resistance of a component or an electrical circuit. The measuring unit is the Ohm, noted Ω.

Principle of operation

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

  1. Measurement of a current with a generator of tension
  2. Measurement of a tension with a generator of current.

Generator of current

A generator of current imposes an intensity IM through unknown resistance RX , one measures the tension VM appearing on its terminals.

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  • Such an assembly does not make it possible to measure with precision of resistances whose value exceeds some kΩ because the current in the voltmeter is not then negligible any more (the internal resistance of the voltmeter being usually equal to 10 MΩ). The assembly is thus supplemented by an auxiliary generator of current controlled to the value of the tension measured by the voltmeter and charged with delivering the current in the voltmeter noted IV

  • When the value of resistance RX is lower than ten ohms, to avoid taking into account various resistances of connection, it is advisable to implement a particular assembly, carried out in the ohmmeters 4 wire.

Generator of tension

One uses a Ampèremètre to measure the current I circulating in a resistance X-ray to which one applies a weak definite tension V .

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  • This method is used in the analogical ohmmeters provided with Galvanomètre to mobile framework.

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