Maker's Name: 
Scintrex Instruments
Where made: 
26.5 × 17 × 7 cm

This unit is used in magnetic surveys to show variations in the Earth's magnetic field, and/or to detect ores by their magnetic signature.

A primary winding is wound on each of two parallel ferromagnetic legs, and fed with AC current. The legs are in anti-phase, magnetically saturated and balanced (equal magnetic fields).

A secondary winding is wound over the two legs.

When an external magnetic field passes through the legs, it unbalances the fields in the legs (reinforcing one and reducing the other). 

The secondary winding detects the differential variation and passes the detected signal to an amplifier and readout.