1. Gregory, R. L., and Donaldson, P. E. K., Nature, 182, 1434 (1958).
2. MacKay, D. M., Brit. Prov. Pat. Spec. 844/49.
3. Teegan, J. A., BEAMA J., 378 (1949).
4. Roberts, F., and Young, J. Z., Proc. Inst. Elect. Eng., 99, IIIA, 747 (1952).
5. Parker, E., and Wallis, P. R., J. Inst. Elect. Eng., 95, III, 371 (1948). MacKay, D. M., Electronic Eng., 248 (July) and 281 (Aug. 1949).