An electrical meter with integral AC current clamp is known as a clamp meter, clamp-on ammeter or tong tester.
In order to use a clamp meter, only one conductor is normally passed through the probe; if more than one conductor is passed through then the measurement would be the vector sum of the currents flowing in the conductors and would depend on the
phase relationship of the currents. In particular if the clamp is closed around a two-conductor cable carrying power to equipment the same current flows down one conductor and up the other, with a net current of zero. Clamp meters are often sold with a device that is plugged in between the power outlet and the device to be tested. The device is essentially a short extension cord with the two conductors separated, so that the clamp can be placed around only one conductor.
The reading produced by a conductor carrying a very low current can be increased by winding the conductor around the clamp several times; the meter reading divided by the number of turns is the current, with some loss of accuracy due to inductive effects.
Clamp meters are used by electricians, sometimes with the clamp incorporated into a general purpose
multimeter.
It is simple to measure very high currents (hundreds of amperes) with the appropriate current transformer. Accurate measurement of low currents (a few milliamperes) with a current transformer clamp is more difficult.
An iron vane type clamp-on ammeter
Less-expensive clamp meters use a rectifier circuit which actually reads mean current, but is calibrated to display the
RMS current corresponding to the measured mean, giving a correct RMS reading only if the current is a
sine wave. For other waveforms readings will be incorrect; when these simpler 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, readings can be quite inaccurate. Meters which respond to true RMS rather than mean current are described as "true RMS".
Typical hand-held
Hall effect units can read currents as low as 200 mA, and units that can read down to 1 mA are available.
The Columbia tong test ammeter, manufactured by
Weschler Instruments, is an example of the iron vane type, used for measuring large AC currents up to 1000 amperes. The iron jaws of the meter direct the magnetic field surrounding the conductor to an iron vane that is attached to the needle of the meter. The iron vane moves in proportion to the strength of the magnetic field, and thus produces a meter indication proportional to the current. This type of ammeter can measure both AC and DC currents and provides a true RMS current measurement of non-sinusoidal or distorted AC waveforms. Interchangeable meter movements can be installed in the clamping assembly to provide various full-scale current values up to 1000 amperes. The iron vane is in a small cylinder that is inserted in a space at the hinged end of the clamp-on jaws. Several jaw sizes are available for clamping around large conductors and bus bars up to 41⁄2 inches (110 mm) wide.
I also have a cheap harbor freight one that was given to me as a gift, but the leads on it are really flimsy so I only use it if my Craftsman isn't handy and I just don't feel like digging for it. I highly recommend the Craftsman one. 
