The null magnetic field as reference for the study of geomagnetic directional effects in animals and man.

Techniques for producing very low and zero magnetic fields are considered, giving attention to the compensation of the geomagnetic field by a Helmholtz coil system, approaches utilizing the shielding power of highly permeable alloys, and the complete exclusion of the geomagnetic field with the aid of a superconductive shield. Animal experiments in low magnetic fields are discussed, together with the exposure of man to 'null' magnetic fields and the Josephson junction as a possible biosensor of magnetic fields. It is found that neither the functions nor the behavior of man changes significantly during a two-week exposure to magnetic fields below 50 gammas.