1. Eddington Space, Time and Gravitation, Harper Torchbooks, p. 94 (1959)
2. P. Laplace, Mechanique Celeste, volumes published from 1799–1825 (English translation reprinted by Chelsea Publ, New York, 1966)
3. R. Becker, Teoria Della Elettricità (Sansoni Ed. Scientifiche, Naples, 1950), pp. 73–77
4. J.D. Jackson, Classical Electrodynamics (Wiley, New York, 1962–1975), pp. 654–658
5. R. Feynman, R.B. Leighton, M.L. Sands, The Feynman Lectures on Physics, vol. II, Chapters 21 and 26.2 (Addison-Wesley, Redwood City, 1989)