Abstract
LONDON. Royal Society, June 10.—Sir Archibald Geikie, K.C.B., president, in the chair.—The functions of the pituitary body (Croonian lecture): Prof. E. A. Schäfer.—A wavelength comparator for standards of length: Dr. A. E. H. Tutton. Two and a half years ago the author was requested by the Standards Department of the Board of Trade to devise and superintend the construction of, a new comparator, for comparing standards of length—the imperial standard yard, for instance, with official copies, and the latter with the copies constructed for local authorities—in terms of wave-lengths of light. The instrument now described is the result. Besides performing its functions, as a wave-length comparator, and being the first instrument, specifically constructed as such, it is also the most perfect instrument yet devised for measurement in wave-lengths in general. It is described to the Royal Society by permission of the President of the Board of Trade. The principle of the instrument is that of the author's interferometer, described to the. society in 1898 in connection with an interference dilatometer, and again as improved in 1904 in connection with, the author's elasmometer or interference elasticity apparatus. The interferometer, which is totally different from that of Michelson or that of Fabry and Perot, is adapted, as regards details, in a special manner for the specific object in view, but with the exception that a Hilger constant-deviation prism is employed instead of a train of two spectroscope prisms, its principle is preserved intact. The essential point of the instrument is that one of the two microscopes, employed to focus the two defining lines on a standard yard bar, actually carries just above the objective one of the two glass plates of the interference apparatus, which reflect the monochromatic light (hydrogen or cadmium red radiation) which is caused to interfere and produce rectilinear dark bands. When the microscope is moved the plate consequently moves with it, and the amount of movement is absolutely afforded by the movement of the interference bands, being equal to half the wave-length of the light employed for every band which passes the reference spot in the centre of the field of the interferometer telescope. So perfectly has this fine move-,ment been achieved that the microscope and the bands can be caused to move simultaneously, by rotation of a large, fine-adjustment wheel, so steadily that each band can be made to pass the reference spot as slowly as one wishes, and be arrested instantly, without the slightest. tremor, at any fraction of its width, so that the control of the bands and the counting is a perfectly simple matter. In order to compare two standard bars, it is only necessary (1) to place the bar of known length, supported on an elaborate mechanism for the adjustment of the bars, also novel, under the two microscopes, carried on massive yet delicately moving sliders on a 6-feet V-and-plane bed, so that the two defining lines are adjusted between the spider-lines of the micrometer eye-piece in each case; (2) to replace the standard by the copy to be tested, so that the defining line near one end is similarly adjusted under the corresponding microscope; then, if the other defining mark is not also automatically adjusted under the second microscope which carries the glass interference plate, as it should be if it is an exact copy, (3). to traverse that microscope until it is so adjusted, and (4) to observe and count the number of interference bands which move past the interference spot during the. process. The difference between the bars is this number multiplied by the half-wave-length of the light in which the bands are produced. The paper also gives an account of the electrical and thermal arrangements, as well as of the foundation masonry of the new comparator room. The temperature of the whole room is controlled entirely electrically, being maintained constant at the official temperature, 62° F. The thermostatic arrangements are of an original character, and of two different independent types—a thermometric and a resistance type.—The use of wavelength rulings as defining lines on standards of length: Dr. A. E. H. Tutton. The delicacy of the method of measurement in wave-lengths described in the preceding communication calls for a corresponding refinement in the engraved lines which form the defining lines of the length of a standard yard or metre or other line-measure bar. The defining lines on the imperial standard yard are sharp-edged, but contain the equivalent of forty interference bands of red light in their thickness, and the Benoit defining- lines of the platinum-iridium copy made in 1902 are not only very ragged-edged, but contain fifteen interference bands in their thickness. The author has been in communication with Mr. J. H. Grayson, of Melbourne, whose fine rulings have recently evoked such interest among microscopists, and after a long1 investigation has found that wonderfully, satisfactory rulings on the scale of 40,000 to the inch can be made on polished speculum metal, covered with a thin cover-glass cemented only at the corners away from the rulings. Now the forty-thousandth of an inch is a single wave-length of red light (for Hα = 1/38710 inch and Cd red = 1/39450 inch), so that the interval between any adjacent pair of these lines is equivalent to only two interference bands. The thickness of each line, which is absolutely sharp-edged, is less than a single interference band. The author has therefore devised a defining mark m these rulings, which he terms a “Tutton location signal,” to distinguish it from the “Benoit defining line.” It consists of five such parallel lines spaced one forty-thousandth of an inch apart, with a pair of strong “finder” lines outside them and parallel to them, and another pair of similar finder lines perpendicularly transverse to them, to indicate a central part of the lines for use. The central line of the five fine Grayson rulings is the defining line. These location signals can also be ruled on platinum-irid'um, and with less success on gold and invar; but the result on speculum metal is so very superior that a large number of location signals have been made on this metal by Mr. Grayson for the Standards Department. The paper indicates their possible mode of use, not only as the end-mark defining lines of standard bars, but for a new mode of determining, by a stepping-off process of reneated doublings, the total number of wave-lengths of red cadmium light contained in the British yard.
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Societies and Academies . Nature 80, 477–480 (1909). https://doi.org/10.1038/080477a0
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DOI: https://doi.org/10.1038/080477a0