A current probe is a device that can measure amperage without breaking a circuit. Current probes can be self contained devices, or they may be designed to work in conjunction with a multimeter. Most current probes consist of a set of clamshell jaws that can be opened in order for a wire to be passed through and then clamped closed. When the jaws are closed, the current probe can use one of a few different methods to indirectly measure any amperage passing through the wire. Most current probes use either a transformer, Hall effect sensor, or iron vane to measure amperage.
Due to the way that electrical circuits work, current can be more complicated to measure than voltage or resistance. Both voltage and resistance can be measured by probing the ends of a component, since voltage remains the same in parallel circuits and resistance is measured directly. Current is uniform in series circuits, so a direct measurement of amperage must involve breaking the circuit. This is not always desirable, or even possible, so current clamps can allow amperage to be measured in an indirect way.
A magnetic field can be generated when electricity flows through a wire, and electricity can be induced when a magnetic field passed through a wire. These principles are used in transformers, and some current probes actually behave like a transformer. This type of current probe contains internal wires that act as a secondary winding, and the wire being measured acts as a primary. An ammeter can then use the current that is induced in the secondary winding to determine the amperage in the wire that is being probed. This type of current probe is only useful when measuring alternating current (AC).
Another type of current probe with a clamp design uses a Hall effect sensor instead of a transformer. These Hall effect probes also use the magnetic field generated by the test circuit to indirectly measure amperage. This is achieved by measuring the voltage difference in an circuit, which is caused by the magnetic field. These current probes are typically very sensitive and able to measure either direct current (DC) or AC.
Iron vane current probes can also measure either AC or DC, and come in a few different configurations. Some iron vane current probes have an integrated meter and clamshell jaws. Other types of iron vane probes also have integrated meters, but lack jaws. These probes are typically held in parallel with the tested wire so that its magnetic field can move the iron vane, which in turn moves the needle on the meter.