Refer to figures.
EASA learning objective 022.04.05.01.02 states: Using a block diagram, list and explain the function of the following components of a remote-reading compass system: flux detection unit; gyro unit; transducers, precession amplifiers, annunciator; display unit (compass card, synchronising and set-heading knob, DG/compass/slave/free switch).

A remote reading compass uses a detector which senses the Earth's magnetic field. This detector is known as a flux detection unit or flux valve and its purpose is to detect the orientation of the horizontal component (H) of the Earth's magnetic field relative to the aircraft.

The flux valve uses 3 detector legs, arranged in two sections, at 120° intervals around a central hub. Wire is wound around the central hub and an alternating current fed through it which produces a magnetic field which reverses every time the current changes direction. This central winding is called the excitation coil.

Secondary coils of wire enclose the 3 upper and lower flux valve detector legs, these are known as pick off coils. It's these 3 pick off coils which sense the strength and orientation of the Earth's magnetic field.