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2. E. Durand, Électrostatique, Vol. II, Masson, Paris, 1966.
3. Addison-Wesley Series in Physics;Panofsky, Wolfgang K. H.,1962
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5. K. J. Binns and P. J. Lawrenson, Electric and magnetic field problems, Macmillan, New York, 1963, pp. 194 and 235