1. W. Kaiser and F. Schwarker, Canad. Patent 974718, (1975)
2. W. MacDonald, S. Park And Chen-yong Pan, U.S. Patent 3,842,515 (1974)
3. P. Manos, U.S. Patent 4,068,387 (1978)
4. Hitachi, Ltd., Jpn. Kokai, 81,07,602 (1981)
5. W.V.Titov, Brit. Patent 1,305,385 (1973)