Anhysteretic Remanent Magnetization due to Asymmetrical Alternating Fields

An instrument is described, suitable for measuring the degree of asymmetry present in the current supply to demagnetizing equipment. Measurements of the natural asymmetry of the AC mains supply at the Close House Palaeomagnetic Laboratory of the University of Newcastle upon Tyne, display a striking daily variation in the percentage asymmetry, and this variation has been found to be consistent over a three-month period. The value ranges from <0.05 per cent in the early morning to >1.5 per cent in the evening. Experiments in which a variety of different rock types were ‘demagnetized’ in the presence of artificially produced asymmetries under controlled laboratory conditions, demonstrate that the levels of natural asymmetry observed in the AC mains supply may be sufficient to produce significant Anhysteretic Remanent Magnetizations (ARMs), unless strict precautions are taken to eliminate them. This paper also describes investigations of certain fundamental properties of this type of ARM and compares these properties with those of ARMs produced by small super-imposed steady fields. It is suggested that ARMs due to asymmetrical fields may be of use in determining such parameters as the distribution of magnetic domains in the a, b plane of the Preisach diagram.