Clamp meters are also known as tong testers.
The most common forms of clamp meter are:
In order to use a clamp meter, the probe or clamp is opened to allow insertion of the wiring, and then closed to allow the measurement. Only one conductor is normally passed through the probe, if more than one conductor were to be passed through then the measurement would be a sum of the currents flowing in the conductors and could be very misleading depending on the phase relationship of the currents. In particuler, if the clamp were to be closed around a mains extension or similar cord, no current will be measured at all as the current flowing in one direction will cancel that flowing in the other direction.
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.
In practice, nearly all clamp meters are used by electricians. It is often assumed in their design that the current is a sine wave of the local mains frequency, ie either 50 or 60Hz. If any of these assumptions are violated, incorrect readings may be obtained.
Better clamp meters make fewer assumptions. In particular, some meters sold by hobby shops may give incorrect readings when used to measure current flowing through inductive loads such as electric motors and the ballasts used with flourescent lighting, owing to the phase relationship between current and voltage. At the other extreme, probes sold for use with some computerised digital multimeters will give accurate readings in almost any situation.