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Older clamp meters are AC devices only. The probe consists of a core of ferromagnetic material, which when closed forms the core of a transformer of which the wiring passing through the clamp is the primary winding. The instrument or probe contains a secondary winding of many turns. By measuring the current induced in this secondary winding, and taking account of the characteristics of the transformer and the frequency and waveform of the current being measured, the size of the current flowing through the conductor can be determined.

Less-expensive clamp meters use an average-detecting rectifier circuit that is then calibrated to read in RMS units; it is assumed in their design that the current is a sine wave of the local mains frequency, that is, either 50 or 60 Hz. If either of these assumptions are violated, incorrect readings may be obtained so when such meters are used with non-sinusoidal loads such as the ballasts used with fluorescent lamps or high-intensity discharge lamps or most modern computer and electronic equipment, the readings produced by such meters can be quite inaccurate.

At the other extreme, probes sold for use with some computerised digital multimeters use true-RMS converters that will give accurate readings in almost any situation. These high-end meters may also replace the transformer secondary winding with a Hall effect sensor that allows accurate readings over a much-wider frequency range, often extending from DC to thousands of hertz and occasionally, for small clamp meters with ferrite cores, extending into the megahertz range.