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4. K. Whitham: Measurement of the geomagnetic elements. Methods and Techniques in Geophysics, pp. 104–167. London: Interscience Publishers Ltd. 1960.
5. R. J. Wold: The Elsec Wisconsin digital recording proton precession magnetometer system. Geophys. and Polar Res. Centre, U. of Wisconsin, Res. Rept. Ser. 64–4, 83 (1964).