1. De Witt, G. US Patent 2 146 210, 1939
2. Schröder, G., Tessmer, K. and Turner, R. H. US Patent 3 284 425, 1966
3. Kopchik, R. M. US Patent 4 246 374, 1981
4. Flexman, E. A. GB Patent 2 101 139, 1983
5. Anzai, I., Makino, H., Sasaki, I., Nishida, K. and Morimoto, M. EP Patent 200 530, 1986