Victor Weisskopf, now at the University of Rochester after fleeing Nazi Europe with Niels Bohr's help, revisits in 1939 a problem he had first tackled five years earlier, in 1934, as Pauli's assistant in Zurich: calculating the electron's self-energy — the energy associated with the electron's interaction with its own electromagnetic field. In that first attempt he made a sign error, spotted by a then little-known physicist at Harvard, Wendell Furry; once corrected, the result showed that the self-energy diverged only logarithmically, in sharp contrast with the linear divergence of classical electrodynamics and the quadratic divergence obtained in a simpler (positron-free) version of quantum theory. When showing the error to Pauli, Weisskopf anxiously asked whether he should give up physics; Pauli, with his characteristic dryness, simply replied: "I never make mistakes." In "On the Self-Energy and the Electromagnetic Field of the Electron" (Physical Review, 1939), Weisskopf extends and rigorously proves that result: he shows that, thanks to Dirac's positron theory — the contributions of virtual electron-positron pairs cancel much of the divergence that would otherwise appear — the self-energy is only logarithmically infinite, and that this result holds to all orders in an expansion in powers of the fine-structure constant, not just in the first approximation. Crucially, Weisskopf clearly recognizes that this remaining infinity, though mathematically present, could be absorbed into a redefinition — a renormalization — of the electron's observed mass and charge, without affecting the theory's verifiable physical predictions. This early recognition of the idea of charge and mass renormalization, a decade before Freeman Dyson generalized the concept to full quantum electrodynamics (1949, see separate entry dyson-qed-equivalence-renormalization-1949), is the specific contribution for which Weisskopf, together with Dyson and 't Hooft, would receive the shared 1981 Wolf Prize in Physics.