In 1910, Arthur Erich Haas proposed a model of the hydrogen atom with an electron circulating on the surface of a sphere of positive charge. The model resembled Thomson's plum pudding model, but Haas added a radical new twist: he constrained the magnitude of the electron's potential energy, , on a sphere of radius to equal the frequency, , of the electron's orbit on the sphere times the Planck constant:

where represents the charge on the electron and