Ohmmetri

Ohmmetri, or an ohmmeter, is an instrument used to measure electrical resistance, typically expressed in ohms. It is commonly a standalone device or a function of a digital multimeter, and is used to assess components, circuits, and wiring by determining how strongly they oppose current flow.

Most ohmmetri operate by applying a small, controlled current to the unknown resistance and measuring the resulting voltage drop, or by applying a known voltage and measuring the current. The resistance is then displayed as R = V / I. In practice, digital and analog meters implement this with an internal current source and a voltmeter, and some designs use a four-wire configuration for low resistances to minimize lead and contact resistance.

Types include handheld digital ohmmetri, analog meters with moving-coil gauges, and benchtop models. The term ohmmetri is also used to describe the resistance-measuring function of a multimeter. For precise low-resistance measurements, four-wire (Kelvin) techniques separate the test leads from the resistance under test to improve accuracy.

Operational considerations: measurements should be performed on de-energized circuits unless the meter is rated for live measurements. Some components may be damaged by the test current, and measurement can perturb the circuit. Calibration and zeroing (shorting leads) are common steps to compensate for the meter’s own resistance and lead resistance.

Applications include testing resistors, checking wire continuity, diagnosing faulty joints, and verifying insulation resistance in equipment. Limitations include accuracy depending on the meter’s range, contact resistance, and test current, especially for very high or very low resistances.

History notes that early ohmmetri used a galvanometer and a stable battery, while modern devices employ digital electronics for faster, more accurate readings.