1. Walmer, M. S., Engelhand Technical Bulletin (1961–1962).
2. Jellinghaus, W. Z., Tech. Phys., 17, 33 (1936).
3. Chaston, J. C., British Patent, 849, 505 (1960).
4. Brissoneau, P., Blanchard, A., and Bartholin, H., I.E.E.E., Trans. on Magn. Mag., 2, 479 (1966).
5. McCurrie, R. A., and Gaunt, P., Phil. Mag., 13, 567 (1966).