1. Chapman , S. J. Bartels Geomagnetism 2 611 612 Oxford University Press London 1940
2. Transactions of the International Astronomical Union Proceedings of the Twelfth General Assembly, Hamburg, Germany, XII B (1964) 594 595 1966
3. Vestine , E. H. L. Laporte I. Lange W. E. Scott The Geomagnetic Field, Its Description and Analysis, Carnegie Inst. Wash. Publ 580 1947