1. Bradley, E. M., J. App. Phys., Supp., 33, No. 3, 1051 (1962).
2. Leaver, K. D., and Prutton, M., J. Elec. Control., 15, No. 2, 173 (1963).
3. Leaver, K. D., Ph.D., thesis, Univ. London (1963).
4. Mossman, P. (to be published).
5. Ruthroff, C. L., Proc. Inst. Rad. Eng., 47, 1337 (1959).