Abstract
The apparatus in precise scanning-wavelength interferometry with gases is described. The dispersion of the refractive index is measured with the aid of an evacuated Michelson interferometer in the wavelength range 400-8 and an uncertainty in the absolute values of mod Delta (n-1)/(n-1) mod =2*10-4. The gas pressure is determined absolutely using a repeatedly calibrated pressure transducer to about mod Delta p mod =5 Pa or mod Delta p/p mod =10-4 in the pressure range 0-3*105 Pa. The temperature is measured with precision mod Delta T mod =1 mK and uncertainty mod Delta T mod =10 mK using platinum resistance thermometers and voltage ratio measurements with the aid of a digital voltmeter. The system of automatic data acquisition is also described.

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