The z-component of the magnetic field at a distance R from a magnetic moment parallel to the z-axis is given by eqn 15.28. In a solid, a proton at a distance R from another can experience such a field and the measurement of the splitting it causes in the spectrum can be used to calculate R. In gypsum, for instance, the splitting in the H20 resonance can be interpreted in terms of a magnetic field of 0.715 mT generated by one proton and experienced by the other. What is the separation of the protons in the H20 molecule?