Measurement of Refractive Indices of Air, Nitrogen, Oxygen, Carbon Dioxide and Water Vapour at 3360 Mc/s

A sensitive and accurate method for the measurement of the refractive indices of gases at 3 360 Mc/s is described and critically examined to assess the systematic errors. The method is based on the measurement of the change of the resonant frequency of a cavity resonator when evacuated and when filled with a gas. A frequency modulation technique is used in conjunction with an oscillograph display. The standard deviation in the measurement of changes of the resonant frequency of the high-Q-factor cavity employed, was 77 c/s; this made possible the determination of (n ¿ 1), where n is the refractive index, to four significant figures. This accuracy was achieved with the aid of a crystal-controlled frequency standard. The results obtained for (n ¿ 1) × 106 at 0° C, 760 mm Hg are as follows: Dry carbon-dioxide-free air 288.26 ± 0.2 Nitrogen 294.47 ± 0.2 Oxygen 266.17 ± 0.2 Carbon dioxide 494.59 ± 0.35 Water vapour (at 20° C and 10 mm Hg) 61.31 ± 0.1 The permanent electric moment of the water molecule was calculated to be (1.845 ± 0.001) × 10¿18 e.s.u. The method of measurement is equally suitable for other microwave frequencies.