1. Sixtus: Physics, 1935, vol. 6, p. 105.
2. Bozorth and Williams: U. S. Patent 2,300,336 dated Oct. 27, 1942.
3. C. G. Dunn: Private communication.
4. Ackermann, et al.: Tech. Mitt. Krupp, 1957, vol. 15, p. 3.
5. Möbius: German Patent 1,009,214 dated May 29, 1957.