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 N his discussion of the electromagnetic mass of a moving electron (Theorie der Elektrizität, ii. p. 205), Abraham raises an objection to the Lorentz conception of an electron as having, at rest, a spherical shape, but in motion the shape of an oblate spheroid the ratio of whose axes is $$\sqrt{1-v^{2}/c^{2}}$$, v being the velocity relative to the tether, and c the velocity