Design and calibration of apparatus for the alternating field demagnetization of rocks

Pearce, George William (1967) Design and calibration of apparatus for the alternating field demagnetization of rocks. Masters thesis, Memorial University of Newfoundland.

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Abstract

Apparatus for demagnetization of rocks in alternating fields up to 800 oe. (peak) has been designed, constructed and calibrated. The unit was tested in detail for effectiveness in removing "unstable" magnetization components., and to confirm the absence of all preferentially directed field components that might cause the specimens to acquire an anhysteretic magnetization. Synthetic materials consisting of one-and two- component powders of magnetite, hematite and pyrrhotite set in a plaster of Paris matrix, as well as natural, basalt, were used in these tests. -- The system was shown to be especially sensitive to direct fields acting parallel to the main (N-S) rotation axis, which is perpendicular to the axis of the demagnetization coils. Spinning of the specimen about three orthogonal axes simultaneously proved superior to a two-axis spin, though alternating field demagnetization even in the highest fields of which the unit is capable, was satisfactory with two-axis spin, provided the direct field in the specimen region was insignificant. -- A number of specimens from rock formations in Newfoundland and Ireland were demagnetized in alternating fields, to test their suitability for palaeomagnetic studies. -- Preliminary attempts to use direct current in the demagnetizing coils showed this method to be feasible, but it requires further tests and some modification to the apparatus before it can be used to advantage.

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