Léon Rosenfeld and Christian Møller related the observed electric quadrupole moment of the deuteron to orientation-dependent nuclear forces. A purely spherical charge distribution gives zero quadrupole moment, because averaging over directions cancels. A measured nonzero value therefore constrains the internal quantum state and the angular dependence of the interaction. Their 1939 work used this small electrical observable to test descriptions of the force between a proton and a neutron.[ref]
The Electric Quadrupole Moment of the Deuteron and the Field Theory of Nuclear Forces by Christian Møller and Léon Rosenfeld. 1939 abstract, deuteron quadrupole moment and orientation-dependent forces; spherical-average observation explained here
Portrait of Léon Rosenfeld
. GFHund. CC BY 3.0, via Wikimedia Commons. Source.
