1. Briggs, B. H., and Spencer, M., “Rep. Prog. Phys.”, 17, 245 (1954).
2. Martyn, D. F., “Physics of the Ionosphere”, 163 (1955).
3. Purslow, B. W., Nature, 181, 35 (1958).
4. Mitra, S. N., Proc. Inst. Elect. Eng., Part 3, 96, 441 (1949).
5. Rao, B. R., Rao, M. S., and Murty, D. S., J. Sci. Indust. Res. India, 15A, 75 (1956).