An Experimental Measurement of the Gyromagnetic Ratio of the Free Electron

The gyromagnetic ratio of the free electron is measured by a method which is an extension of the classical double-scattering experiment. A magnetic field is interposed between the first and second scattering foils, whose direction is parallel to the path followed by the electrons. The electron spins precess in the magnetic field, resulting in a rotation of the plane of maximum asymmetry, as observed after the second scattering event. In the experiment reported, the rotation is approximately 1800 degrees. In the motion of the electron between the two scatterers the small lateral component of velocity gives rise to a "cyclotron" motion whose frequency is, theoretically, the same as the spin precession frequency to within about one part in a thousand. The cyclotron motion, therefore, furnishes a convenient reference frequency, but it also introduces problems in that it causes the asymmetries which have their origin in geometrical misalignment, finite aperture, etc., to follow the rotation of the spin asymmetry. By comparing all measurements made with the foils of high atomic number with measurements made with an aluminum foil of equal scattering power, and by further precautionary procedures and cross checks, the spin asymmetry is separated from asymmetries of other origin. The result, for 420-kev electrons and gold scatterers, is $g=2.00\mathrm{}\pm 0.01$. Plans for a more precise measurement are mentioned.