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A description is given of a mass spectrograph designed to work with gases at very low pressures. The metal parts are made entirely of tantalum and tungsten enclosed in an all glass system which may be baked out quite thoroughly, making it possible to work with gases at pressures of the order of 10^-6 mm. The apparatus has been applied to the study of the ionization potential of molecular hydrogen yielding a value of 15. 370. 03 volts. A discussion of all the previous measurements of this quantity is given.
Walker Bleakney (Sun,) studied this question.