ohmmeter

An ohmmeter is an electrical test instrument used to measure electrical resistance, typically expressed in ohms (Ω). It is commonly part of a multimeter or a standalone device used in electronics and electrical work to identify faulty components, test cables, and verify circuit connections.

Ohmmeters apply a small, controlled electrical current to the component under test and measure the resulting voltage. Using Ohm's law, R = V/I, the instrument computes and displays the resistance. Analog ohmmeters rely on a moving-coil meter and a battery, while digital ohmmeters use solid-state circuitry and a digital display. Many include a zero or short-circuit adjustment to compensate for lead resistance.

Most devices provide multiple resistance ranges or auto-ranging. For accurate readings, the circuit must be de-energized and the component removed from a live circuit; in-circuit measurements can be distorted by parallel paths. Temperature and contact resistance can affect results, as can a degraded test lead or weak batteries. Low-resistance measurements require careful technique and may use a dedicated low-resistance method.

Insulation resistance tests require a megohmmeter, which applies a higher voltage and uses a dedicated insulation tester rather than a standard ohmmeter. Ohmmeters are used to test resistors, fuses, wiring, cables, and components during manufacturing, repair, and troubleshooting. They should not be used on powered circuits unless the device is designed for live measurements and safety guidelines are followed.