By D. Collinson
During the final 30 years the research of the magnetic houses of rocks and minerals has considerably contributed to a number of fields of technology. might be the easiest recognized and most vital advances have resulted from the examine of palaeomagnetism, which resulted in quantitative affirmation of continental waft and polar wandering via interpretation of the course of remanent magnetism saw in rocks of alternative a while from various continents. Palaeomagnetism has additionally, via observations of reversals of magnetiz ation, historical secular version and historical box intensities supplied info proper to the beginning of the geomagnetic box, and different investigations have contributed considerably to large-scale and native geological stories, the courting of archaeological occasions and artefacts and extra lately to lunar and meteoritic reports. Rock and mineral magnetism has proved to be an attractive examine in its personal all over the advanced magnetic homes and interactions saw within the iron-titanium oxide and iron sulphide minerals, in addition to contributing to our knowing of remanent magnetism and magnetization tactics in rocks. Simultaneous with the improvement of those reports has been the improve ment of tools and methods for the big variety of investigations involved.
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Extra resources for Methods in Rock Magnetism and Palaeomagnetism: Techniques and instrumentation
Sample text
Any of the methods of measuring susceptibility in an alternating field can, in 34 Methods in Rock Magnetism and Palaeomagnetism principle, be used to measure the variation of susceptibility with frequency, although there may be practical difficulties. For instance, in bridge circuits because of dependence of the coil inductance on frequency, and in balanced transformer circuits because of the need to preserve good voltage compensation in the double-coil system over a range of frequencies. The latter problem was encountered by Likhite and Radhakrishnamurty (1965) and they used a small variable capacity across one of the double-coil windings to compensate for the variation with frequency of the phase difference between the two voltages.
2 Transformer bridge circuit (Girdler, 1961). a, air-cored coils; b, Mumetalcored coils; c, oscillator; d, detector. moving a closed wire loop parallel to the axis of one of the coils, and inductive balance is obtained by positioning a small ferrite slug inside one of the coils. The change in inductance of the other coil when a rock sample is placed in it unbalances the bridge and it is rebalanced by adjustment to the ferrite slug, the necessary displacement being proportional to the susceptibility of the sample.
When a sample is placed in or near the inner coil, the change in flux linkage with that coil is greater than with the outer coil and a net induced voltage appears across the coils. This type of instrument has close affinities with the spinner magnetometer, and the exciting field can be considered as inducing a sinusoidally varying magnetic moment in the sample which then induces the alternating voltage in the coil system. s. 19) where v is the sample volume, B is the inducing field, and w its angular frequency.