Abstract: A pair of semi-circular, hard magnetic sensors are rigidly fixed within separate blocks which are in turn removably received in corresponding housings mounted to the jaws of a spring biased clamp. The clamp is adapted for positioning the sensors into a split ring around a wire carrying the DC current to be measured. According to a first embodiment of the method, longitudinally wound coils about each of the sensors are momentarily energized by an AC current source connected thereto through a variable transformer, while the sensors are around the wire. Thereafter the sensors are removed from the clamp and placed side by side beneath a compass so that the magnetic flux previously induced in the sensors deflects the compass needle from North. Circumferentially wound coils about each of the sensors are connected to a DC current source which is varied to move the compass needle back to North by cancelling the magnetic flux previously induced in the sensors.

Abstract: The present invention contemplates an improved active differential filter network comprised of a bridge network having a pair of input-output junctions and a pair of control junctions, and an operational amplifier having an inverting input connected to one of the control junctions and an output connected to the other control junction, the operational amplifier having a non-inverting input connected to a bias control signal.