ohmmetre

An ohmmetre is a device used to measure electrical resistance. It can be a dedicated instrument or the resistance-measuring function of a multimeter. In many cases it applies a small, known test current to the unknown resistance and measures either the resulting voltage or the current to determine resistance using Ohm's law, R = V/I. Modern digital ohmmetres typically display a direct resistance readout, while analog models use a calibrated scale.

Handheld ohmmetres cover ranges from a few ohms to many megohms. Some instruments employ four-terminal (Kelvin) connections to minimize lead resistance and improve accuracy. Measurements are generally performed with the circuit unpowered to avoid damage and to prevent readings being distorted by other circuit paths.

Calibration and accuracy depend on the instrument's internal reference, temperature, and the condition of test leads. Zero or short-circuit adjustments are common on older models to account for lead resistance; higher-end devices may offer traceable calibration against standards. Temperature effects and battery health can influence results, so readings may vary with environmental conditions.

Limitations include in-circuit measurements, where parallel paths or energized components can skew results. High-resistance measurements can be affected by stray capacitance or inductance at higher ranges, producing transient or inaccurate readings. Ohmmetres are essential in electronics for diagnosing faulty components, validating resistor values, and verifying circuit integrity.