A spinner magnetometer

Abstract

A spinner type rock remanent magnetometer is described. The instrument meas- ures specimens having moments from 2 X 10 -6 emu upward, and measures moments from 10 -5 emu with precisions of 1 ø in direction and 5 per cent in magnitude. The lower limit for measurement corresponds to an intensity of magnetization of 5 X 10 -s emu cm -s for the largest size of specimen which can be accommodated. A discussion is given of the factors which limit the sensitivity of a spinner magnetometer. Annulment of the geomagnetic field at the rotor is shown to be desirable when rocks having low remanence and high susceptibility are to be measured. Introduction. Measurements of the rema- nent magnetization of rocks have become im- portant in connection with paleomagnetic re- search. Such measurements are made either by means of some type of astatic magnetometer (Blackerr, 1952) or by means of a spinner or rock-generator type of magnetometer. Spinner magnetometers, in which the rock specimen is rotated relatively slowly on a shaft, have been described by Johnson et al. (1948), by Buck- shaw and Robertson (1948), and by Gtiffiths (1955). The compressed-air turbine described by Beams (1938) is used to give much faster rotation in spinner magnetometers described by Graham (1955) and by de Sa and Molyneux (1963). In 1954 I constructed a simple spinner mag- netometer with a Beams compressed-air tur- bine, which was used in a paleomagnetic study of the Pilansberg dikes (Gough, 1956). The Beams turbine was adopted from the instru- ment later described by Graham (1955). The magnetometer here described was designed and constructed in 1957 and has been in use ever since in paleomagnetic studies in southern and central Africa. More than four thousand meas- urements have been made with it, and minor modifications and improvements to the design have been added over the 6-year period. The instrument has therefore proved itself, and it