1. Schottky, W., Ann. Phys., 57, 541 (1918).
2. For example, Williams, F. C., J. Inst. Elec. Eng., 78, 326 (1936).
3. v. Laue, M., Jahrb. d. Rad. u. Elek., 15, 205 (1918).
4. Möller, H. G., and Detels, F., Jahrb. d. draht. Teleg. u. Telep., 27, 74 (1926).