In 1908, O. W. Richardson showed that if the magnetic properties of materials are due to the rotation of the electron orbits of the atom, a change Ul in the angular momentum should be accompanied by a change in magnetic moment Ml which is given by Ul/Ml = 2m/e. (1) The angular momentum thus associated with the change of magnetic moment is very small, and has hitherto been measured only for ferromagnetic bodies. Experiments by various workers using different methods have shown that the angular momentum is within the limits of experimental error exactly one-half that expected on the orbital electron theory. This has been known as the gyromagnetic anomaly. Recent theoretical developments, however, indicate that the source of magnetic moment in ferromagnetic bodies is due to the spin of the electron. For the spinning electron, the relationship between change of magnetic moment and angular momentum is Us/Ms = m/e, (2) this equation being well established on the experimental side, principally from the Zeeman effect.