1. Weyl, H. (1950).Space-Time-Matter. Dover Publication.
2. Møller, C. (1962).The Theory of Relativity, Oxford University Press.
3. Landau, J. L. D. and Lifschitz, E. M. (1961).The Classical Theory of Fields, Pergamon Press.
4. Bertotti, B. (1966).Proc. Roy. Soc. A,269, 195.
5. Balazs, N. L. (1957).Phys. Rev.,110, 236.