Refer to figure.
The purpose of the flux valve in a helicopter is to detect the direction of the Earth's magnetic field and provides aircraft heading by measuring the local direction of the horizontal component of the Earth's magnetic field, relative to the aircraft's orientation.

A flux valve provides information electronically rather than mechanically. Therefore, it does not need to be near the flight deck. The flux valve in a helicopter is typically positioned in the tailboom section of the fuselage. This placement is chosen to minimise deviation errors by keeping it as far as possible from interference from the aircraft's own magnetism.