Before we go on to consider proposed solutions to the problems posed by the continuous energy spectrum in β decay, we must consider some of the closely related problems associated with the structure of the atomic nucleus. Physicists at the time thought, correctly, that several of these problems might have the same solution. In 1930, the time we are considering, the only elementary particles known to exist were the proton, the nucleus of the hydrogen atom; the electron; and the photon, the corpuscle of electromagnetic radiation. In 1911 Ernest Rutherford had proposed the nuclear model of the atom on the basis of a-particle-scattering experiments performed by Geiger and Marsden. In this model, almost the entire mass of the atom was contained in a very small (compared to the size of the atom), positively charged nucleus composed of protons and electrons. 1 The overall electrical neutrality of the atom was provided by electrons surrounding the nucleus. The charge of the nucleus, at least for light elements, was half its atomic weight, in units of the weight of the hydrogen atom. For helium, which had a mass of 4 and a nuclear charge of 2, this led to a nucleus composed of 4 protons and 2 electrons. For nitrogen, with mass 14 and charge 7, there were 14 protons and 7 electrons. As Niels Bohr remarked, “The empirical evidence regarding the charges and the masses of these nuclei, as well as the evidence concerning the spontaneous and the excited nuclear disintegrations, leads, as we have seen, to the assumption that all nuclei are built up of protons and electrons” (Bohr 1932, p. 379).