1. For example, see, The Economist, V305, N 7527, 94 (1987);
2. Research & Development, V. 30, N. 9, 3 (1988), New York Times, May 17, p26 (1988)
3. W. M. Winslow, J. Appl. Phys. 20, 1137 (1949); U.S. Patent 2, 417, 850 (1947).
4. H. Block and Kelly, US Patent 4,687,589 (1987);
5. J.D. Carlson, US Patent no. 4,772,407 (1988)