1. Millikan and Cameron, Phys. Rev., 31, 922; 1928.
2. Millikan and Cameron, Phys. Rev., 37, 237; 1930.
3. Hoffmann in (5) below comments upon having also observed this relation in his own earlier work.
4. Broxon, Phys. Rev., 37, 1320; 1931.
5. Hoffmann, Zeit. für Phys., 69, 704; 1931.