1. T. A. Edison, U.S. Pat 1,016,874 (1912).
2. R. L. Tichenor, U.S. Pat. 2,578,027 (1951).
3. A. Dassler, U.S. Pat. 2,104,973 (1938).
4. A. E. Lange, E. Langguth, E. Breuning and A. Dassler, U.S. Pat. 2,131,592 (1938).
5. G. F. Rublee, U.S. Pat. 2,269,040 (1942).