1. Ger. Pats. 480 866 (July 20 1924) and 507 962 (Apr. 30 1927) W. O. Herrmann and W. Haehnel (both to Consortium für Electrochemische Industrie).
2. U.S. Pats. 1 955 068 (Apr. 17 1934) andGer. Pat. 683 165 (Oct. 12 1939) H. Hopff and E. Kuehn (both to I.G. Farbenindustrie).
3. U.S. Pat. 2 036 092 (Mar. 31 1936) RE20 430 (Reissued June 29 1937) G. O. Morrison F. W. Skirrow and K. G. Blaikie (to Canadian Electro Products Co.).
4. U.S. Pat. 2 114 877 (Apr. 19 1938) R. W. Hall (to General Electric Co.).