1. Hollmann, Ann. Phys., 86, 129, 1928.
2. Tank and Schiltknecht, Helvetica Phys. Acta., 1, 110, 1928.
3. See, for example, Möller, Jahrb. drahtl. Tel., 34, 201, 1929.
4. Paper to appear shortly, probably in Jour. I.E.E.
5. Potapenko, Z. tech. Phys., 10, 542, 1929.