1. P. F. Smith, J. D. Lewin, C. R. Walter, M. N. Wilson, and A. H. Spurway, J. Phys. D3, 1517 (1970).
2. A. R. Kaufman and J. J. Pickett, Bull. Am. Phys. Soc. 15, 833 (1970).
3. K. Tachikawa, Int. Cryo. Engin. Conf., Berlin (1970); Iliffle Sci. Tech. Pub. 339 (1971).
4. E. W. Howlett, U.S. Pat. 3,728,165 (filed Oct. 19, 1970); a Great Britain Pat. 52, 623/69 (filed Oct. 27, 1969).
5. C. D. Henning, IEEE Trans, on Magnetics MAG-15 525 (1979).