Measurement techniques and the multimeter

A multimeter is the most common instrument for electrical measurement, combining several functions in one device. At minimum it acts as a voltmeter to measure voltage, an ammeter to measure current, and an ohmmeter to measure resistance, with the user selecting the quantity and range. Modern instruments are digital, displaying a numeric reading, while older analog meters indicate the value with a moving pointer driven by a galvanometer, a sensitive current-detecting movement.

Each measurement mode connects the meter to the circuit differently. A voltmeter is placed across the points whose potential difference is wanted and must have a high input resistance so it draws little current. An ammeter is placed in series so the current flows through it and should have very low resistance. An ohmmeter passes a small known current through a component and infers resistance from the resulting voltage. Related instruments such as power meters combine voltage and current measurements to report power.

Measuring alternating signals introduces subtleties. The meaningful value of an AC waveform is usually its root-mean-square (RMS) value, which corresponds to the equivalent heating effect of a DC value. Simple meters estimate RMS by assuming a pure sine wave, but a true-RMS meter computes the real RMS value and so reads correctly for distorted or non-sinusoidal signals. The crest factor, the ratio of peak to RMS, indicates how spiky a waveform is and how demanding it is to measure accurately.

Frequently asked questions

How should a multimeter be connected to measure current?

To measure current the meter is connected in series with the circuit so the current flows through it. To measure voltage it is connected in parallel, across the points of interest.

What does true-RMS mean?

A true-RMS meter calculates the actual root-mean-square value of a signal, giving correct readings even for distorted or non-sinusoidal waveforms, unlike simpler meters that assume a pure sine wave.

What is crest factor?

Crest factor is the ratio of a waveform's peak value to its RMS value. A high crest factor indicates a spiky waveform that is harder to measure accurately.