The Bates Saccharimeter and Its Operation
For the adjustable halfshade analyzer of his saccharimeter, Bates chose to use a large and small Nicol-prism combination that had been developed previously, but used only to give a fixed halfshade angle. The rotation of one prism relative to the other would provide a range of halfshade angles. Bates added a geared device to maintain the two prisms in proper position when adjusted so the halves of the field would be at equal intensities at the end-point the halfshade angle selected. When his saccharimeter was finally built and tested, it exhibited no observable change in the zero point over a halfshade angle range from 2.5º to 15º, and it proved to be capable of measuring sucrose rotations with high precision.
The knob facing the observer is used to select the half shade angle, which is indicated on the dial located above that knob. The knobs to the right and left of the face plate turn the left- and right-rotating quartz wedges, respectively. On the axis of each of the two knobs there are a worm gear and a worm that can be swung into engagement to provide a slow movement for a fine setting. Either knob may be clamped in a fixed position. Rotations are observed through the two eyepieces above the main eyepiece. The scales are ruled on glass and viewed by transmitted light, enabling readings to be made without parallax. A system of mirrors reflect light from the main lamp to illuminate the scales. The scales are graduated in accordance with the International Sugar Scale and the least difference of reading is approximately one-tenth that of the older type saccharimeter. The saccharimeter is checked for accuracy by reading a standard quartz plate which gives a known rotation of the plane of polarization corresponding to a known value of the sugar scale. A thermometer is located at the top of the faceplate to show the measurement temperature, so the rotations may be corrected to a standard temperature. For the most precise work, it is advantageous to filter the white light used as the light source through a solution of potassium chromate, which absorbs the blue portion of the spectrum and lessens the effect of the small discrepancies in the equality of the dispersions of sucrose and quartz.
The saccharimeter can be used with a monochromatic light source for measuring other optically active substances; hence, it is equipped with two adjustable quartz wedges. Measurements made in the International Sugar Scale are converted into circular degrees with the factor appropriate for the wavelength of the monochromatic light used.
Photos: Polarimetry, Saccharimetry and the Sugars by Frederick J. Bates and Associates, Circular of the National Bureau of Standards C440, May 1942
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