A small sensitive magnetometer

A magnetometer is described which was designed primarily for measuring a magnetic field within the confined space of a permeameter. The measuring head of the instrument contains a thin solenoid through which a direct current is passed, to balance the field being measured and produce zero field in the solenoid. A sensitive field detector within the solenoid consists of a fine ferromagnetic (Mumetal) wire carrying an alternating current of frequency 5 kc/s. This current develops a saturating circular field in the wire and causes its effective longitudinal permeability to alternate at twice the frequency of the current. A field along the wire thus produces an alternating induction in the wire and consequently an alternating e.m.f. in the solenoid; this e.m.f. therefore serves as a sensitive indication of field. With associated equipment designed especially for the magnetometer it is possible to detect a change in field strength of 2 × 10¿5 oersted. Alternatively the head can be used with standard laboratory equipment allowing a field of 2 × 10¿3 oersted to be detected. The maximum field strength that can be measured is limited to about 50 oersteds by the heat developed in the solenoid by the balancing current.