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We have carried out the largest search for stable particles with fractional electric charge, based on an oil drop method that incorporates a horizontal electric field and upward air flow. No evidence for such particles was found, giving a 95% C.L. upper limit of $1.17\times 10^{-22}$ particles per nucleon on the abundance of fractional charge particles in silicone oil for $0.18 e \le |Q_{residual}| \le 0.82 e$. Since this is the first use of this new method we describe the advantages and limitations of the method.Comment: 11 pages, 9 figures, 3 table

For use in our Millikan type liquid drop searches for fractional charge elementary particles we have developed a simple ionized air device for neutralizing a narrow stream of small drops. The neutralizer has been used for drops ranging in diameter from 10 to 25 µm. The width of the produced charge distribution is given by the Boltzmann equilibrium charge distribution and the mean of the distribution is set by a bias voltage.Using the bias voltage, the mean can be set with a precision of better than e, the electron charge. The use of the neutralizer is illustrated in an application to mineral oil drops produced with charges of the order of 1000e. We also show the interesting case of silicone oil drops that are produced in our drop generator with a charge distribution narrower than the Boltzmann equilibrium charge distribution, the charge distribution being broadened by the neutralizer.

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Several techniques for determining the size of small fluid microdrops with diameter ranging from 5 µm to 30 µm have been developed and evaluated using an automated variation on the Millikan oil drop experiment. The average diameter of a large sample of monodisperse fluid drops was determined by measuring their terminal velocity in air, or if charged, their motion under the influence of an electric field, as well as by measurement of the magnitude of their Brownian motion. The diameter of individual drops was determined optically, by direct observation using an imaging system based on a coupled device (CCD) camera. The technique used to analyze the image data is based on a best fit technique taking the point spread function (PSF) of the lens into account, and yields results accurate to 1% (based on a single image) without the need for any calibration. By combining this technique with terminal velocity measurements, the density of the fluid can be determined to similar accuracy.

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