Page:EB1911 - Volume 18.djvu/406

 micrometer with the upper side of the box removed. The letters in the description refer to both figures.

S is the head of the micrometer screw, s that of the screw by which the micrometer box is moved relative to the plate f (fig. 8), s′ that of the screw which moves the eyepiece slide. K is the clamp in position angle, P the slow motion screw in position-angle; pp is the position circle, R, R its two readers. The latter are in fact little microscopes carrying a vernier etched on glass, in lieu of a filar micrometer. These verniers can be read to 1′, and estimated to 0′·2. D is the drum-head which gives the fraction of a revolution, d that which gives the whole number of revolutions, I is the index or pointer at which both drums are read. This index is shown in fig. 9, but only its mode of attachment (X, fig. 9) in fig. 8. The teeth of the pinion z, fig. 9, are cut on the axis of the micrometer screw.

EB1911 - Micrometer - Fig 9.png . 9.

The drum d and its attached tooth wheel are ground to turn smoothly on the axis of the screw. The pinion z and the toothed wheel d are connected by an intermediate wheel and pinion Y; the numbers of teeth in the wheels and pinions are so proportioned that twenty-four revolutions of the micrometer screw produce one revolution of the drum and wheel d. The divisions of both drums are conveniently read, simultaneously, by the lens e; at night the lamp which illuminates the webs and the position-circle also illuminates the drum-heads (see on illumination p. 385). is the web-frame (fig. 9), is a single rod consisting of two cylinders accurately fitting in the ends of the micrometer box, the larger cylinder being at. There is a hole in the webframe which smoothly fits the larger cylinder at ′, and another which similarly fits the smaller cylinder at ′. A spiral spring, coiled round the cylinder, resting one end on the shoulder formed by the difference of the diameters of the cylinders and  and the other on the inside of the web-frame, presses the latter continuously towards. Contact of the web-frame of the micrometer with the side of the box at would therefore take place, were it not for the micrometer screw. This screw fits neatly in the end of the box at, passes loosely through the web-frame at ′, is tapped into the frame at ′, and its end rests on a flat hardened surface at. Rotation of the web-frame about is prevented by the heads of the screws at m; the head of the screw on the lower side of the frame reposes on the plane, that on the upper side (fig. 9) touches lightly on the inner surface of the lid of the box. Such rotation can obviously be controlled within limits that need not be further considered. But freedom of rotation in the plane of the paper (fig. 9) is only prevented by good fitting of the holes ; and, since the weight of the slide is on one side of the screw, misfit here will have the effect of changing the reading for coincidence of the movable with the fixed web in reverse positions of the micrometer. With the Cape micrometer a systematic difference has been found in the coincidence point for head above and head below amounting to 0″·14. This corresponds, in the Cape instrument, with an excess of the diameters of the holes over those of the cylinders of about th of an inch—a quantity so small as to imply good workmanship, though it involves a systematic error which is very much larger than the probable error of a single determination of the coincidence point. The obvious remedy is to make all measures on opposite sides of the fixed web before reversing in position-angle—a precaution, however, which no careful observer would neglect. In measuring differences of declination, where the stars are brought up by the diurnal motion, this precaution cannot be adopted, because it is necessary always to bisect the preceding star with the fixed web. But in measures index error can be eliminated by bisecting both stars with the same web (or different webs of known interval fixed on the same frame), and not employing the fixed web at all. The discordance in zero, when known to exist, is really of no consequence, because the observations can be so arranged as to eliminate it.

The box is mounted on a strong hollow steel cylinder CC (fig. 9) by holes, in the ends of the box, which fit the cylinder closely and smoothly. The cylinder is rigidly fixed in the studs C, C, and these are attached to the foundation plate f. The cylinder contains towards a sliding rod, and towards  a compressed spiral spring. There is thus a thrust outwards of the spring upon the hollow cap W (attached outside the box), and a thrust of the rod upon the end of the screw s. The position of the box relative to the plate f, in the direction of measurement, depends therefore on the distance between the end of the screw s and the fixed stud C. A screwing in of s thus causes the box to move to the left, and vice versa. Rotation of the box round CC is prevented by downward pressure of the spring Z on a projection attached to the side of the box. The amount of this pressure is regulated by the screw z′.

The short screw whose divided milled head is shifts the zero of the micrometer by pushing, without turning, the short sliding rod whose flat end forms the point d’ appui of the micrometer screw at. The pitch of the screw is the same as that of the measuring screw (50 threads to the inch), and its motion can be limited by a stolp to half a revolution.

The five fixed webs are attached to the table, which is secured to the bottom of the box by the screws. The three movable webs are attached to the projections on the frame. The plane surfaces and  are composed of a bronze of very close texture, which appears capable of receiving a finish having almost the truth and polish of an optical surface. It seems also to take a very clean V cut, as the webs can be laid in their furrows with an astonishing ease and precision. These furrows have apparently been cut in situ with a very accurate engine; for not the slightest departure from parallelism can be detected in any of the movable webs relative to the fixed webs. Extraordinary care has evidently been bestowed in adjusting the parallelism and distance of the planes and, so that the movable wires shall almost, but not quite, touch the surface. The varnish to fix the webs is applied, not on the surface as is usual, but on a bevel for the purpose, the position of the webs depending on their tension to keep them in their furrows. The result is that no trace of “fiddling” exists, and the movable and fixed webs come sharply together in focus with the highest powers. Under such powers the webs can be brought into apparent contact with such precision and delicacy that the uncertainty of measurement seems to lie as much in the estimation of the fraction of the division of the head as in the accuracy of the contact.

EB1911 - Micrometer - Fig 10.png . 10.

It is a convenient feature in Repsolds’ micrometer that the webs are very near the inner surface of the top of the box, so that the eye is not brought inconveniently close to the plate when high powers are used.

Another excellent micrometer, originally based on a model by Clark of Cambridge, Massachusetts, has been largely used by Burnham and others in America. The form, as constructed by Warner and Swasey for the 40-in. Yerkes telescope, is shown in figs. 10 and 11. The micrometer box, and of course with it the whole system of spider webs, is moved by the screw s, whilst the measuring web is independently moved by the screw S. The other parts of the instrument will be readily understood from the figure without further explanation. The method of counting the total number of revolutions gives more friction and is less convenient than Repsolds’, and no provision seems to be made for illuminating the micrometer head in the practical and convenient plan adopted by Repsolds.

Repsolds’ more recent form of the spider-line micrometer (since