A current sensor for measuring current through a conductor with the current sensor including a cable looped around the conductor to be monitored, and a transformer winding disposed in the cable for measuring current flow based on the Rogowski principle. The cable has opposite end portions coupled in an abutting relationship to form an endless loop with the winding flush with opposite end faces of the opposite and portions of the cable. The current sensor also has a coupling in the form of a sleeve including an assembly for attaching and detaching the opposite end positions of the cable having a stop ring which grips one of the end portions and a push button which is selectively movable against the stop ring for releasing the grip to open the opposite end portions. The cable may have a temperature sensor and a memory in which the measurement characteristics of the sensor are held. The sensor is reliable and accurate, and it can be left permanently in an electrical apparatus, or replaced, with no difficulty.