Micrometer

MICROMETER, a name generally given to any device for measuring small angles or dimensions (from Gr. yucp6s, small, pfrpov, a measure). In particular a great variety of appliances used for astronomical measurement are called micrometers.

One of the immediate difficulties of accurate measurement of linear dimensions is to secure coincidence between the measuring appliance and the object measured. For inaccessible (celestial) objects, coincidence is, of course, impossible; for accessible ob jects, e.g., a photographic plate, the plate and the measuring appliance are likely to get in each other's way. The remedy is that either the measuring scale or the thing measured should be insubstantial. Certain devices, such as interferometers, can be regarded as representing the first alternative ; but, in general, the simplest solution is to arrange that the thing measured shall be insubstantial, viz., an optical image, accessible and offering no interference with our measuring tools, but forming a faithful reproduction to scale of the object whose dimensions we really wish to know. Both in the telescope and the microscope a real image is formed in the focal plane ; here we place the "pointer" of our micrometer—a movable wire, a scale ruled on glass, or some other fiducial mark—and move it about in the midst of the ghost that we are measuring. We view the coincident pointer and image through an eyepiece, which acts as a magnifying glass. Most micrometers make use of the principle of the screw; the pointer is displaced uniformly by turning a screw. If, for ex ample, the step of the screw is o.5 mm., and the screw-head is read to of a revolution, we measure to o.000s mm. (about equal to the wave-length of light). Needless to say, many precau tions are necessary if we would actually attain such high accuracy.

Filar

first micrometer used with a tele scope was made by William Gascoigne, about 1638. It consisted of two pointers with parallel straight edges, which could be brought together or separated by turning a screw. He made measurements of the diameters of the sun, moon and planets, which are still extant, and prove that his invention made a very great advance on earlier measurements. The modern successor of this instrument is the filar micrometer, in which the two pointers are replaced by parallel wires or spider-webs, carried on metal frames. Two screws are provided, one of which moves the pair of wires as a whole, and the other alters the separation of the two wires. In most observations, only the reading of the second screw is required, the first screw being merely an aid in setting. Much work formerly done by visual measurement is now done photographically, and the use of the filar micrometer is now mainly for measurement of the separations and position angles of double stars. (See also HELIOMETER.) Travelling-wire Micrometer.—This is an appliance used in conjunction with the transit circle (q.v.) which has proved very successful in avoiding magnitude-equation and other systematic errors, and is now generally adopted in fundamental observation. The older method was to tap off on a chronograph the observed times of passage of a star across a series of fixed wires set in the focal plane of the telescope. In the travelling-wire or "imper sonal" micrometer, the observer continually turns a screw so as to keep a fine wire or web continually bisecting the moving star image. (The turning of the screw moves the eye-piece as well as the wire, so that the bisected star remains apparently station ary at the centre of the field of view.) Electrical contacts are

made when the moving frame reaches definite positions, and the time when the wire, and theref ore the star, reaches a series of positions is thus automatically recorded on the chronograph.

Photographic Measuring Machines.

These are examples of the application of the micrometer to the microscope. There is a wide variety of patterns, and nearly every observatory has its own form, designed to embody some special improvement, valu able either for securing accuracy or for saving time in the par ticular work there pursued. Many lines of work involve a prodigious amount of measurement of photographs, and the time-saving factor is therefore of great importance. We can only summarize here some of the typical requirements. The photographic plate must be carried on slides allowing it to move in two perpendicular directions, so that any point of it can be brought directly under the microscope. The frame should have great rigidity, in order that perfect focus may be preserved. The cross-wires, glass scale or other fiducial marks, are in the focal plane of the microscope objective, and are movable in two direc tions at right-angles by two micrometer screws. The eye-piece is mainly responsible for the magnification. Magnification by the objective is seldom greater than 3 :1; there are, in fact, many advantages in using unit magnification, i.e., the image in the focal plane of the objective is the same size as the photograph itself. It may be recalled that in visual observing we magnify with the eye lens an image formed in the focal plane of the telescope ; by in serting a photographic plate in the focal plane this image is materialized and rendered permanent ; the objective of the measuring microscope converts this back into an insubstantial image, preferably of the original size, and we then make our deferred examination of it with an eye-lens.

In accurate (as distinct from time-saving) machines, the modern tendency has been towards simpler construction. The secret of success is to remove the difficulties before reaching the stage of micrometric measurement, instead of elaborating the measuring instrument to solve them. In researches on stellar parallax and proper motion, it is necessary to compare two or more photographs of star fields taken at different epochs. In parallax work the photographs are now often taken on the same plate (which is kept undeveloped in the interval between the epochs), the different images of the same star being arranged close to gether in a symmetrical way. In proper motion work the photo graph at one epoch is taken through the glass of the plate, i.e., with the plate "the wrong way the two plates to be corn pared are then placed film to film so that corresponding images are very near together. In either case the comparison resolves itself into measuring exceedingly small distances between pairs of images, and there are comparatively few sources of error.

Similar principles apply to machines for measuring spectra (celestial or laboratory). Here the motion and measurement are in one dimension only, and a line is a somewhat easier object to set on, either with single or parallel wires, than a star image. Whenever possible the measurements are made relatively to a comparison spectra, so that only small differences of position are to be measured. (A. S. E.)