In the displacement type of internal measuring machine, due to J. E. Sears, the position of the ring is so adjusted that measure ments are made exactly across the diameter. The ring is mounted on a carriage, which can be moved bodily in the direction of the diameter being measured by means of suitable micrometers. Con tact is made first on one side of the ring, and then on the other, with a double-ended stylus attached to a sensitive indicating mechanism, and the micrometer readings are taken when the indi cator reads zero. The same is done in turn with a standard plug substituted for the ring, provision being made for withdrawing the stylus temporarily as the plug passes across. If yi x2 y2 are the four micrometer readings, the displacements and are equal to xi and yi —y2 respectively, and it is to be noticed that the pressure on the stylus is in the same direction when both x read ings are taken, and also when both y readings are taken, so that any possible errors due to backlash or flexure in the indicating mechanism are eliminated. The transition from external to internal measurement is thus directly accomplished, and the final result is given by the equation Scientific Principles.—The question of the general geometri cal principles underlying the correct design of instruments in tended to give the highest accuracy of measurement has received greater attention in recent years. Questions of the proper appli cation of the theory of kinematic constraint to ensure definite and repeatable registration of parts, of eliminating backlash and fric tion effects, of preventing components of errors due to inevitable imperfections of workmanship (e.g., in sliding ways) from be coming effective in the actual direction of measurement, of the design and relationing of parts so that elastic deformations due to changing distributions of load do not affect the results, and so on, are all involved here.
These various points have been fully considered in the travel ling microscope comparator designed by J. E. Sears, jun., for the Metrology Department of the National Physical Laboratory. The long leg of an L-shaped carriage, which bears two microscopes, is supported on two wheels running in a V-guide along the upper side of the machine bed, so that whatever position is taken up by the carriage its form is undisturbed. The focal points of the two microscopes are arranged to lie in the extension of the axis of the micrometer screw, and the object to be measured is supported on an independent carriage which, by means of a small weight at tached to a cord passing over a pulley, is held permanently in contact with a stud at the end of the screw abutment. Thus errors of straightness in the V-groove are non-effective, and so is any distortion of the base of the machine proper, due to the move ment of the microscope carriage. Advantage is taken of the simple ratio i in. = 25.4 mm. (which is correct to within 2 parts in ,000,000) to obtain simultaneous readings of equal accuracy in either British or the metric system, by means of suitable gearing in the compound micrometer head. The verniers read to in. and o.000i mm. respectively. The instrument can be used either with one microscope for making direct measurements against its own calibrated micrometer screw, or, using the two microscopes as a comparator, for determining the values of the sub-divisions of short scales.
War the necessity for rapid measurement of large numbers of screw gauges led to the development of special machines for the measurement of both pitch and effective diameter of screw threads. E. M. Eden, then of the National Physical Laboratory, was largely instrumental in this work and in the development of the optical projection method of examining profiles of gauges, both screws and flat templets. For the latter, he found lens combinations capable of giving an undistorted magnification of so times over a field of initial diameter approaching two inches. These combina tions were incorporated in the now well-known "horizontal pro jector." For screw gauges the "vertical projector" was designed, in which the path of the light is vertical, and the image-forming rays are reflected back from an optically flat mirror overhead on to a specially prepared thread-form diagram placed on the table of the machine. The screw in this case is mounted in a carriage provided with two horizontal micrometer traverses in directions parallel and perpendicular to its axis, respectively, so that measure ments of pitch and diameter can he made at the same time as the accuracy of the thread form is examined. Mention should also be made of the Wilson projection gauge, in which the optical system is duplicated, so that the two opposite sides of a screw can be projected simultaneously on to the screen, in such a manner that the two images of the thread intermesh, and if the screw is of direct form and size, and the apparatus correctly adjusted, exactly meet. If the screw is small a space is left between the images of its two sides; if large they overlap.
The projection method has naturally proved of great value in a number of other ways. In particular, it has been used in con nection with the measurement of gear wheels and gear cutting hobs. Machines for this purpose have been designed by G. A. Tomlinson, and depend for their use on the accurate reproduction of the profiles of the teeth of the gear or hob, in the form of traces made on smoked glass, by a needle point attached to a specially designed pantograph. One of the machines is also fitted with a device for recording, in a similar manner, the relative ve locities of rotation of two gear wheels when in mesh. In both cases the smoked glass, bearing the record, is put into the projection apparatus and magnified so times at the screen. It is found that the traces are quite sharp in the magnified image, and measure ments can be made corresponding to an accuracy of about o•000r or 0•0002 inch on the original.
BIBLioGRApHY.—Annual Reports of the National Physical Labora tory, Stationery Office, London; R. T. Glazebrook, Dictionary of Applied Physics, vol. 3. Articles on "Design of Scientific Instru ments," "Gauges," "Meters," "Metrology"; E. A. Griffiths, Engineer ing Instruments and Meters (1921) ; R. S. Whipple, "Design and Construction of Scientific Instruments," Trans. Opt. Soc., 22, No. 2, 1920-21 ; C. G. Peters and H. S. Boyd, "Interference Methods for Standardizing and Testing Precision Gauge Blocks," Bureau of Standards, Scientific Papers, No. 436; A. F. C. Pollard, "The Mechanical Design of Scientific Instruments," Cantor Lectures, Royal Society of Arts, 1922 ; J. E. Sears, jun., "Precise Length Measure ments," Cantor Lectures, Royal Society of Arts, 1923. (J. E. S.)