Abstract: An electromagnetic device is disclosed for use in a polygraph instrument for moving a recording pen in response to sensed electric current. The device includes a movable magnet that is rotated by a magnetic field produced by the sensed current flowing through the coil of an electromagnet. Null magnets are releasably held at selected positions and in a particular arrangement relative to the movable magnet and have their respective magnetic field strengths and positions such that they automatically return the movable magnet to a null position when the current being sensed is removed from the coil.