The FX-220 Flux Valve is a detector that transmits magnetic information as electrical energy. The Flux Valve contains a pendulous sensing element mounted in a damping fluid. Universal mounting permits this sensing element up to 30 degrees freedom in roll and pitch, while prohibiting rotation about the vertical axis. The sensing element consists of a transformer with one primary and three secondary windings, wound about a spider-shape core. The secondary windings are positioned 120 degrees apart in a horizontal plane. The primary winding is located at the center of the spider-shape core.

The FX-220 Flux Valve has a 3.58 inch bowl diameter, mounting flange diameter of 4.80 inches, height of 2.88 inches and weighs 1.50 pounds maximum. The FX-220 has a power requirement of 23.5 ± 0.05 V, 400 Hz, 0.04 amp single-phase. The FX-220 detects the direction of the lines of force of the earth's magnetic field and transmits this information electrically to the directional gyro. When a current is passed through a coil of wire, a magnetic field is set up as much as lines of flux flow easier through ferrous metals than through air, the core in the center of the coil concentrates the magnetic field and thus increases the field density. To control the induced magnetic field so that it may be used within a flux valve. In the FX-220 the metal core is called the spider core.