1. Smythe, W.R.: Static and dynamic electricity, (McGraw Hill 1950), p. 20
2. Maxwell, J.C.: A treatise on electricity and magnetism, 3rd edn(Oxford University Press 1892),2, reprinted 1955
3. Heaviside, O.: Electromagnetic theory, 3rd edn(Chelsea Pub. Co., New York 1971),with preface by Whittaker, E.T.