A synchrotron to accelerate protons to 1 GeV was constructed in the Department of Physics at the University of Birmingham and operated from 1953 to 1967. The beam circulated in a rising magnetic field and the frequency of the accelerating voltage had to rise from 0.3 to 10 MHz in synchronism with this rising field. The frequency was controlled by varying the capacitance of a tuned circuit, and this disc, designed by L. U. Hibbard, rotating at 60 Hz in an air-spaced capacitor controlled the capacitance. Image P0215b shows it in situ with the cover removed. Its profile was carefully cut by hand, and the series of tiny mirrors in he black ring on its surface was used to monitor and so control its rotation. In later years of operation the system was replaced by an analog computer.