The Valency of Photo-Electrons and the Photo-Electric Properties of Some Insulators

Photo-emission from insulators; the valency of photo-electrons.—Charged droplets of different insulators are suspended in the field of a parallel plate condenser by regulating the strength and direction of the electric field so that the electric force on the droplet is equal and opposite the gravitational force. Ultra-violet light of various intensities and frequencies is allowed to fall on the droplets and the photo-emission is observed. The photo-emission of several insulators is studied in this manner. By making the light intensities sufficiently low, the author finds that only one electron escapes at each emission. This is true for all insulators studied and is verified in the case of each insulator by the observation of several hundred emissions. This result is in line with similar work on ionization of gases by X-rays, $\gamma$-rays, $\beta$-particles, and $\alpha$-particles by Professor Millikan and his students. By the use of different absorption screens the long wave-length limit of photo-emission from sulphur, shellac, oil and paraffine is located. These are located with greater accuracy than given by any other methods. The effect of water vapor and various surface impurities on photo-emission is discussed.