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Abstract In a previous paper by the author* it is shown that the general theory of quantum mechanics together with relativity require the wave equation for an electron moving in an arbitrary electromagnetic field of potentials, A0, A1, A2, A3 to be of the form FΨ ≡ p0 e / c A0 + α1 (p1 + e / c A1) + α2 (p2 + e / c A2) + α3(p3 + e / c A3) + α4mc Ψ = 0. (1) The α’s are new dynamical variables which it is necessary to introduce in order to satisfy the conditions of the problem. They may be regarded as describing some internal motion of the electron, which for most purposes may be taken to be the spin of the electron postulated in previous theories. We shall call them the spin variables.
P. A. M. Dirac (Thu,) studied this question.