Ant,

A fluxgate magnetometer uses a primary winding around a core to drive the core in and out of magnetic saturation. A secondary winding is used to detect the position change of an ambient field as the core is driven in a out of saturation. In modern realizations the core is a toroid, the primary winding is a toroidal coil, the secondary winding is wound across the top down one side across the bottom and up the other side of the toroid.

When the toroid is saturated, the ambient field will not penetrate the toroid as it is magnetically 'hard'. As current in the primary winding changes direction, the toroid comes out of saturation and the ambient field begins to penetrate the core. The component of ambient field aligned with the secondary coil axis of symmetry will induce a voltage in the secondary coil. The voltage will increase in magnitude as the ambient cuts across the seconday coil until it is aligned with the coil. The primary coil current continues to increase driving the toroid into saturation in turn pushing the ambient field away from the secondary coil thereby reducing the voltage induced in the secondary coil. This whole thing continues as long as the primary coil is driven by ac current sufficient to cause saturation in the core.

A modification to the basic instrument described above is to add a nulling coil whose winding axis is aligned with the secondary (or sense) coil above. The modification provides for sensing of very small ambient fields.

The sense coil voltage will be dependent on (1) ambient field strength, (2) number of sense coil windings &, (3) primary drive frequency.

Engineering considerations are (1) amount of primary drive current necessary to cause saturation, (2) sense coil winding alignment error causing cross axis ambient sensitivity and primary drive induction, (3) frequency content of ambient field to be measured & (4) co-alignment of sensee coil and nulling coil if used.

Cheers,

Bruce