An infra-red spectrograph having sufficient flexibility to permit a wide variety of investigations is described. Its main characteristics follow. Any one of two prisms and two gratings, mounted on a table rotated by a large worm wheel, may readily be put into the path of the collimated radiation. A foreprism spectrometer is built into the instrument in such a way that it may be by-passed. The spectrograph case and the source housing are evacuable. Moving parts in the evacuated chamber are controlled through induction and Autosyn types of motors. This permits control, entirely electrical, from a semi-portable unit outside the spectrograph. The widths of the slits are automatically varied so that the output of the radiation detector thermocouple is approximately the same for all spectral regions if no absorbing material is in the path. The radiation detector is a rapid-response evaporated thermocouple whose output is amplified by a vacuum tube amplifier. A pulsating thermocouple output voltage is obtained by interrupting the radiation at the entrance slit by a rotating shutter. Shutters of metal, mica, and sodium chloride are used to minimize the effect of scattered radiation. To adjust the frequency of the shutter to match the sharply tuned amplifier, the rotational speed of the shutter disk is controlled through a synchronous motor driven by a variable-frequency power supply. A thyratron-operated calibration flasher is used to put reference lines on the photographically recorded spectra. The housing in which the Nernst glower is mounted is equipped with windows so that it may be divided into compartments to serve also as a vapor cell. This unit carries a mounting designed to locate a liquid cell precisely.
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