1. P. Weiss and A. Preuss, Trans. Faraday Soc. 8 (1912) 154.
2. G. W. Elmen, US Patent 1,739752 (Applied 12/28/27) December 17 1929.
3. J. H. White and C. V. Wahl, US Patent 1,862,559 (Applied 8/14/31) June 4 1932.
4. G. A. Kelsall and E. A. Nesbitt, US Patent 2,190,667 (Applied 4/9/38) February 20 1940.
5. E. A. Nesbitt and G. A. Kelsall, Phys. Rev. 58 (1940) 203.