Abstract: A "clip-on" probe for measuring electric currents by operation of the Hall effect comprises a ring of magnetizable material, at least part of which is mounted in a set of opposed jaws (4,5). The jaws are movable between an open position in which the jaws can be placed over a conductor and a closed position in which the jaws meet around the conductor and the ring is substantially complete. To facilitate removal of the probe when a heavy current is flowing through a conductor between the jaws, the jaws are provided with an opening mechanism with an actuator (7) so constructed and arranged that initial pressure on the actuator urges the jaws (4,5) apart from the closed position at a first mechanical advantage and, after an initial movement of the actuator, subsequent pressure urges the jaws towards the fully open position at a second lower mechanical advantage.