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A Precision Two Crystal X‐Ray Spectrometer of Wide Applicability with Worm Wheel Drive; an Improved Precise Method of Equalizing the Spacing of Worm Wheel Teeth
1.A. H. Compton, Rev. Sci. Inst. 2, 365 (1931).
1.The instrument used by Compton for this work, constructed by the Société Génevoise, had two excellent divided circles one of which, however, since it controlled the motion of the ion chamber arm was hardly in a position to serve to the best advantage. In Compton’s design the x‐ray tube must be rotated about axis A which may be a disadvantage as S. K. Allison later pointed out. On the other hand an instrument by Gaertner Scientific Corporation built for Allison has the same scheme of motions as the one here described but is not adapted for absolute glancing angle measurements since with it “angular measurements of rotation through large angles about axis B cannot be made with great accuracy.” See S. K. Allison, Phys. Rev. 41, 1 (1932).
2.DuMond and Hoyt, Phys. Rev. 36, 1702–1720 (1930). This spectrometer has all the requisite motions although it is used with a moving x‐ray tube and a fixed detecting system including an ion chamber mounted directly above a Hoffman electrometer. It has the great angular sensitivity, etc., required for the study of rocking curves but absolute values of large angles cannot be measured with high precision.
3.T. R. Cuykendall and M. T. Jones, R.S.I. 6, 356 (1935).
3.These authors have a two crystal spectrometer design for the range using a sine screw arrangement similar to that of F. K. Richtmyer and S. W. Barnes, R.S.I. 5, 351 (1934). While excellent for shorter wave‐lengths it cannot be used for negative or mixed orders as these authors point out nor will it serve for Compton’s method of measuring absolute angles. The instrument of Richtmyer and Barnes permits all measurements and uses except the last mentioned.
4.The tube could, it is true, have remained stationary without necessitating a stationary beam by mounting crystal A on a carriage translating on a slide midway between focal spot and crystal pivot B. This clever scheme is due to P. A. Ross while an equally ingenious unpublished scheme of P. Kirkpatrick permits both stationary source and ion chamber window. The angle of emergence of the x‐rays from the target surface however changes with the setting; in both of these schemes unless the tube is movable.
5.In the paper by DuMond and Hoyt already referred to in reference 2 our method of aligning crystals so that the intermediate beam from crystal A to crystal B shall be parallel to the line of centers of the crystal pivots is fully explained, pp. 1715, 1716. The optical reflection method serves to give correct settings generally closer than one minute of arc so that little time is lost in “finding a line” the first time with the x‐rays. Bearden has recently announced that a comparison of optical and x‐ray reflections from cleavage planes fails to agree precisely as to the orientation of the reflecting surfaces.
6.This hole facilitates observing optical reflections with a Gauss eyepiece from both sides of a parallel optical flat temporarily mounted in place of the crystal.
7.We are indebted to F. K. Richtmyer for pointing out to us after this work was completed the following references to the use of the split wheel and dowel pin design: Handbuch der Astronomischen Instrumentenkunde 1, 145 (1899),
7.the American Machinist 20, 531 (1897);
7.21, 303 (1898)., Am. Mach.
8.The writer of the article in the American Machinist, 21, 303 (1898), states in no uncertain terms that the strict equality of spacing of the four dowel pins in his wheel was essential to the process of tooth error correction!
9.The idea of extending our lapping schedule by this approximate observance of the formula only occurred to us after the worm wheels here described were completed and tested. It could not be applied to them as they had no Tee slots. We have every reason to believe it sound and it will be applied on the worm wheels for driving the 200‐inch telescope. The principle should be of great utility since it permits the designer to give his wheel any number of teeth he wishes (though by so doing he may of course be obliged to sacrifice the convenience of the use of the dowel pins).
10.One is at first tempted to analyze the actions of lapping schedules on the error graph by expressing this graph as a Fourier series. We believe, however, that the method here presented is simpler and easier.
11.This makes advisable provision for centering the wheel on its pivot by very minute shifting after it has been lapped—a precaution unfortunately not observed in the present design. The splitwheel method of course only equalizes the tooth spacing correctly with respect to the axis defined by the rotation of the ring on the wheel.
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