Metal detecting device with magnetically influenced Hall effect sensor

The combination comprising a pair of permanent bar magnets and a solid state Hall effect sensor including input and output leads, the magnets and sensor being disposed in fixed spatial relationship and held in assembled relationship in a binder, the sensor being located between the magnets in a position wherein the broad plane of the sensor is parallel to an exposed surface of the binder and the magnets having their axes mutually perpendicular but parallel and perpendicular respectively with respect to said plane of the sensor in positions wherein flux from the magnets at least partially cancelling in the sensor, and a ferrous element relatively movable with respect to the sensor and magnets in a direction parallel to the broad plane of the sensor and effectively thereby to control voltage across the output leads during the application of a constant voltage to the input leads.