An Ionization Gauge for Measuring Very Low Pressures.—This gauge is the result of an attempt to develop a more convenient instrument than those previously available. It is based upon the fact that the positive ionization produced by a given stream of electrons varies with the gas pressure. By adopting a construction similar to that used for low power pliotrons, erratic effects due to the charging up of the glass walls were avoided and, with a constant electron emission, a linear relation was obtained between ionization current and pressure down to the lowest pressure actually obtaiued, about ${10}^{-3\mathrm{}}$ bar. For pressures above ${10}^{-1\mathrm{}}$ bar the calibration was carried out by comparison with a McLeod gauge; but for lower pressures a method was used which involves an application of Knudsen's laws of flow. Characteristic curves showing the effect of varying the positive voltage and the electron emission on the ionization current are given. The low limit of pressure measurable with such a gauge is fixed only by the sensitivity of the galvanometer used to measure the ionization current. With an ordinary sensitive galvanometer, pressures as low as ${10}^{-4\mathrm{}}$ bar can readily be determined.

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