Fig. 138.—Grubb's plan of securing micrometer screw.
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343.—It is very general in instruments at the present time to tap the micrometer screw directly into the micrometer frame, and to make the drum and milled head a part of the screw. In this case a very soft motion may be given to the screw by dividing its nut longitudinally and bringing the parts together with a certain amount of spring. Sir Howard Grubb, of Dublin, has placed a spring ball fitting, as shown Fig. 138 at EE′, over the screw upon his astronomical instruments, which gives a very soft motion to the screw. These refinements are very important, as it is not desirable that any undue pressure should be put upon a delicate instrument which under all conditions must be made rigid enough to resist it, and the greater the pressure required to bring the instrument to bearing the stronger it must be made.
Fig. 139.—Stanley's micrometer slide.
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344.—Stanley's Micrometer.—The author has made an arrangement in which the screw has a long, double tubular sliding stem, Fig. 139. The inner stem which carries the milled head has a groove cut down it, into which a stud from the inside of its covering tube slides. This arrangement permits the milled head to be pressed inwards or outwards in turning it without any pressure coming upon the micrometer greater than the friction upon the sliding tube, and that of a weak spring which keeps the stem nearly extended in its tube. A simple Hook's joint H is formed at the head of the screw, so that no part of the weight of the hand comes upon the screw. A tubular guard-piece T prevents the milled head hanging down too far when out of use. When the screw is used it is lifted to about the centre of the guard tube. With this arrangement, as no practical weight or pressure comes upon the micrometer from handling it, the supporting frame-work may be made much lighter than is necessary with any other form of micrometer.
345.—The author prefers to form the micrometer scale and the index of fine lines engraved upon parallel worked glass for surveying instruments. This avoids the risk of breaking webs, and, what is much more important, he finds that with engraved lines on glass he is able to bring the scale and index exactly and permanently into the plane of mutual foci of the object-glass and eye-piece by placing the lines upon the same faces of glass, thus avoiding the great difficulty of focussing to guess-work of an intermediate position between two sets of webs at different distances.
The strip of glass A is fixed by a clamp and two screws to the side of the micrometer box. The slip B is ground and polished to fit A. B is carried by the micrometer frame F, which holds it in a clamp by two screws. A spring, not shown, presses B against A, so that any displacement of the micrometer lines may be made by the milled head. The lines upon A are adjusted to the position of the circle they are intended to read at exactly 5′ or other quantity.