Absorption coefficient of Hg vapor for electrons with energies from 0.5 volts to 400 volts.—An account is given of measurements by two different methods of the absorption coefficient $\alpha$ in mercury vapor for electrons with energies of 0.5 volts up to 400 volts. The experiments described in the first part of the paper were made by Ramsauer's method, in which a homogeneous electron beam is obtained by bending the electrons through a series of slits suitably arranged in a magnetic field. The second part of the paper deals with experiments made by an arrangement which gave a fairly homogeneous beam of electrons without the use of a magnetic field. In each method the fraction of the initial electrons which traversed a given distance through the vapor without suffering collision was measured, and this enabled $\alpha$ to be calculated. The results obtained by the two different methods were found to be in fair quantitative agreement. The absorption coefficient increased continually as the energy of the electrons was reduced down to 0.5 volts. No evidence was obtained of the sharp decrease in $\alpha$ for electrons slower than 3 volts as reported by Beuthe. The results when compared with those obtained by former workers were found to be in fairly good agreement with those of Maxwell over the whole range of electron energies and with those of Brode at the lower energies.

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