Residual Charge of Niobium Spheres

In 1964, Gell-Mann and Zweig hypothesized that the particles inside nuclei are composed of quarks with electric charges of (+OR-) 1/3 e and (+OR-) 2/3 e. Since then, many searches have been made for stable fractional charges. The experiment described is a Millikan-type electrometer with a sample 10('7) times heavier. A 0.28 mm diameter superconducting niobium ball is levitated magnetically between capacitor plates. The ball oscillates vertically on this magnetic spring and an electric field is applied at the oscillation frequency. The rate of change of the ball's oscillation amplitude is proportional to the force exerted on the ball by the electric field. The force on the ball is measured as a function of its position between the capacitor plates to discriminate against the background force due to the patch effect field gradient of the plates. There is one other background effect, due to the tilting of the ball's magnetic moment, which has sometimes manifested itself as a drift or occasionally as an offset in the measured force. An analysis presented in this thesis explains a mechanism for this effect and shows that we can eliminate it in our next run by spinning the ball about the vertical axis. The results are strong evidence for the existence of stable fractional charges of (+OR-) 1/3 e in matter. Out of 58 measurements on 20 balls, 9 have yielded results near - 1/3 e, 34 near 0, 13 near + 1/3e, one of 0.19 e, and one of 0.44 e. Out of 38 repeat measurements, there have been 12 residual charge changes, 5 near - 1/3e and 7 near + 1/3 e.