We have devised and demonstrated the successful operation of a low-cost, high-mass throughput technique capable of performing bulk matter searches for fractionally charged particles based on an improved Millikan liquid drop method. The method uses a stroboscopic lamp and a charge coupled device video camera to image the trajectories of silicone oil drops falling through air in the presence of a vertical, alternating electric field. The images of the trajectories are computer processed in real time, the electric charge on a drop being measured with an rms error of 0.025 of an electron charge. This error is dominated by Brownian motion. In the first use of this method, we have looked at 5 974 941 drops and found no evidence for fractional charges in 1.07 mg of oil. With 95% confidence, the concentration of isolated quarks with $\pm \frac{1}{3}e$ or $\pm \frac{2}{3}e$ in silicone oil is less than one per 2.14×${10}^{20}$ nucleons.

• Received 27 December 1995

DOI:https://doi.org/10.1103/PhysRevD.53.6017