Experience shows that atomic clocks exhibit sharp spectral lines that are unaffected by their history in the presence of a magnetic field

As London (1927, 376–377) remarks, one must admire Weyl’s immense courage in developing his gauge invariant interpretation of electromagnetism and holding on to it on the mere basis of purely formal considerations. London observes that the principle of equivalence of inertial and gravitational mass, which prompted Einstein to provide a geometrical interpretation of gravity, was at least a physical fact underlying gravitational theory. In contrast, an analogous fact was not known in the th