ohmmeters

Meaning: An ohmmeter measures the resistance of a component. Its internal battery drives a small current through the component, and the meter converts that current into a resistance reading. Because it supplies its own power, the component should be isolated from the circuit's source and from parallel branches, or the reading is distorted.

Equation: R = V/I, where R is resistance in ohms (Ω), V is the meter's internal battery voltage in volts (V), and I is the measured current in amperes (A).

On the MCAT: Passages contrast meters. An ammeter goes in series and ideally has very low resistance; a voltmeter goes in parallel and ideally has very high resistance; an ohmmeter goes across a de-energized, isolated component. Questions may ask what it reads across a network, using the series rule (Req = R₁ + R₂) and the parallel rule (1/Req = 1/R₁ + 1/R₂).

Common confusion: Students connect an ohmmeter like a voltmeter across a resistor in a live circuit. The circuit's own source then adds current and spoils the reading. An ohmmeter makes its own current, so the circuit's source must be off.