A clamp meter (clamp-on ammeter) is a type of ammeter, which measures electrical current without the need to disconnect the wiring through which the current is flowing. Clamp meters are also known as tong testers or Amprobes, one of the first vendors of such devices. The most common forms of clamp meter are, a probe for use with a multimeter, a self-contained unit, a built-in part of a specialized multimeter used by electricians. 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 vector sum of the currents flowing in the conductors and could be very misleading depending on the phase relationship of the currents.