Dielectric Constant of Water Vapor

The frequently discussed deviations from linearity in the curves of (K-1) (K+2) versus pressure for water vapor have been investigated. It has been shown: (1) That with proper location of the insulators of the vapor condenser in weak fields, the dielectric constant curves for the vapor are linear until saturation is approached closely; they are linear over the entire range of pressures at which breaks have previously been reported; (2) that neither pronounced breaks similar to those reported by several workers, nor the small residual deviations always present near saturation are due to the conductivity effect as commonly supposed; (3) that these breaks are due wholly to the added polarization contributed by the film of water molecules adsorbed on the insulator surfaces. Accurate dielectric data have been obtained for water vapor at 14 temperatures ranging from 21. 3^ to 197. 9^; they can be represented by the equation (K{-1) (K+2) RT}{p^'}= (4. 030. 39) + (20, 710140) T. These data fall accurately along a Debye line, yielding an electric moment of (1. 831 0. 006) 10^-18 e. s. u. for the water molecule.