The Dielectric Constant of Water Vapor in the Microwave Region

The dielectric constant of water vapor was determined at 9280 Mc by a cavity comparison method at 10 temperatures ranging from 32 to 103°C. In addition, observations were made at 24,800 Mc at the single temperature of 24.5°C. Although the vapor pressure was raised to within 10 percent of the saturation value at 24.5°C and 103°C, the variation of (ε′−1)/(ε′+2) with pressure remained linear, contrary to observations made at radiofrequencies. These data are discussed in relation to the questions of association and adsorption of the vapor. The present results are in agreement with those obtained at radiofrequencies (sufficiently below saturation pressures) and do not show the dispersion reported in an early microwave experiment. Results obtained here and those obtained by others are used to determine mean values for the constants in the Debye equation: namely, P=3.96±0.32+(2.077±0.016)104/T. This gives a dipole moment of (1.846±0.005)10−18 esu.