The measurement of the magnetic properties of rock specimens

Abstract

A method of measuring non-uniformly magnetized specimens by means of an astatic magnetometer is described. The first section gived the theory of the design of a two-magnet system. It is shown that the magnet assembly should be designed for maximum efficiency under the restrictions that are imposed by the physical and magnetic properties of the magnet, the magnet assembly and the specimen. The design of a magnetometer with a period as short as 13 seconds and yet sensitive enought to measure accurately most sedimentary rocks is given. The sensitivity can be varied by changing the suspension and can possibly be increased by a factor of 20. It is concluded that, in general, it is preferable to use transversely magnetized magnets. This section also gives a brief description of a magnetometer capable of measuring extremely weak magnetic fields: 1.2x10-10 oersted. The magnetometer built and used at the Dominion Observatory is described in the second section. This section also gives a bried description of fluxgate system that permits the taking of measurements on magnetically disturbed days. Specimens of cylindrival form are often used for measurements with an astatic magnetometer. The last section shows that the response of the instrument depends on the relative dimensions of the specimen, i.e. on the ratio of height to radius. While is has been standard practice to use thin discs, it is concluded that the ratio of height to radius should be approximately equal to 2.5.