The Dielectric Constant of Water Vapor at a Frequency of 42 Megacycles

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Abstract
A heterodyne beat apparatus was constructed for the measurement of the dielectric constant of water vapor at 42 megacycles. One of the oscillators was crystal-controlled, the other having a tuned circuit containing a condenser which could be evacuated and filled with water vapor. A cathode-ray oscilloscope was used to indicate zero beat. Drift of the oscillators was eliminated from the results by plotting readings on a time base and alternating between vacuum and water vapor as rapidly as physical limitations would permit. Air was used as the calibration gas, because it is composed of nonpolar molecules. It should therefore have the same dielectric constant at this frequency as at the lower frequencies at which it has been measured by other observers. The Clausius-Masotti relation was verified for water vapor at constant temperature, and the dielectric constant of water vapor determined at three different temperatures and 760 mm of mercury. The temperatures in question were 71.9°C, 99.8°C, and 147°C, at which the dielectric constant was found to have the values 1.0071, 1.0060, and 1.00475, respectively.

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