Simplified circuit diagram for an ohmmeter. A represents the ammeter, Rf the fixed resistor, Rv the variable resistor, and Rt the resistor under test.

An ohmmeter is an instrument for providing a rapid, if approximate, value for the resistance of part of an electric circuit. It consists of an ammeter (reverse calibrated in ohms) in series with a fixed resistance, a battery, and a variable resistor. This last is adjusted to zero the meter with the terminals of the instrument shorted. The test resistance is then introduced between these terminals. The reduction in the current so caused is a measure of the resistance.