1. P. Jung and J. van Cakenbergne, Arch. Sci. (Geneva) 14, 132 (1961).
2. L. R. Alldredge and I. Saldukas, The Automatic Standard Magnetic Observatory: US Technical Bulletin No. 31, Department of Commerce, Environmental Science Services Administration, Coast and Geodetic Survey (GPO, Washington, 1966).
3. A. P. De Vuyst, Report to the General Assembly of the International Association of Geomagnetism and Astronomy, Moscow, 1971.
4. A. J. Fairweather and M. J. Usher, J. Phys. E 5, 986 (1972).
5. M. J. Usher and J. P. Reid, J. Phys. E 11, 1169 (1978).