The micrometer shown in fig.
Micrometer with the upper side of the box removed.
The micrometer is clamped in FIG.
Repsolds' more recent form of the spider-line micrometer (since 1 The marks of varnish so applied will he seen in fig.
It became, in fact, essential to invent a " micrometer " for measuring the small angles which were thus for the first time rendered sensible.
The micrometer so mentioned fell into the possession of Richard Townley of Lancashire, who exhibited it at the meeting of the Royal Society held on the 25th of July 1667.
A steel cylinder (about the thickness of a goose-quill), which forms the micrometer screw, has two threads cut upon it, one-half being cut with a thread double the pitch of the other.
Huygens, in his Systema saturnium (1659), describes a micrometer with which he determined the apparent diameters of the principal planets.
The Marquis Malvasia in his Ephemerides (Bologna, 1662) describes a micrometer of his own invention.
A still further facility was given to the use of the filar micrometer by the introduction of clockwork, which caused the telescope automatically to follow the diurnal motion of a star, and left the observer's hands entirely at liberty.'
The micrometer by Trough ton, fitted to their 5 ft.
Equatorial telescope, is the first position micrometer constructed capable of measuring position angles to 1' of arc.
The micrometer represented in fig.
5 is the original Merz micrometer of the Cape Observatory, made FIG.
For the internal structural details of the micrometer the reader is referred to the article " Micrometer " in the 9th edition of the Encyclopaedia Britannica.
5) the eyepiece can be made to follow the star for a considerable distance along a position-web parallel to the screw, the bisection of the web by a star moving by the diurnal motion at right angles to the micrometer screw can only be followed for a limited distance, viz.
Dawes, who employed a micrometer of the English type (figs.
The end-plane of this cylinder receives the pressure of the micrometer screw, so that by turning the small drum-head the coincidence-reading of the movable web with the fixed web can be changed, and thus any given angle can be measured with different FIG.
The same firm is also constructing a micrometer in which the readings of the head are printed on a band of paper instead of being read off at the time of observation.
Instruments have been invented by Alvan Clark and Sir Howard Grubb for measuring with the spider-line micrometer angles which are larger than the field of view of the eyepiece.
The micrometer of Auzout and Picard was provided with silk fibres or silver wires instead of the edges of Gascoigne, but one of the silk fibres remained fixed while the other was moved by a screw.
Beyond the introduction of the spider line it is unnecessary to mention the various steps by which the Gascoigne micrometer assumed the modern forms now in use, or to describe in detail the suggestions of Hooke, 4 Wren, Smeaton, Cassini, Bradley, Maskelyne, Herschel, Arago, Pearson, Bessel, Struve, Dawes, &c., or the successive productions of the great artists Ramsden, Troughton, Fraunhofer, Ertel, Simms, Cooke, Grubb, Clarke and Repsold.
For the measurement of wider stars he invented his lamp-micrometer, in which the components of a double star observed with the right eye were made to coincide with two lucid points placed io ft.
When equatorial mountings for telescopes became more general, no filar micrometer was considered complete which was not fitted with a position circle.'
The use of the spider line or filar micrometer 1 In 1782 (Phil.
Dr Hooke made the important improvement on Gascoigne's micrometer of substituting parallel hairs for the parallel edges of its original construction (Hooke's Posthumous Works, p. 497).
They are placed at a distance apart less than the focal length of a, so that the wires of the micrometer, which must be distinctly seen, are beyond b.
The slides are accurately fitted so as to have no sensible lateral shake, but yet so as to move easily in the direction of the greatest length of the micrometer box.
One of the most essential points in a good micrometer is that all the webs shall be so nearly in the same plane as to be well in focus together under the highest powers used, and at the same time absolutely free from " fiddling."
The English micrometer still retains the essential features of Troughton's original construction above described.
S is the head of the micrometer screw proper, s that of the screw moving the slide to which the so-called " fixed web " is attached, s' that of a screw which moves the eyepiece E.
The later English artists have somewhat changed the mode of communicating motion to the slides, by attaching the screws pdrmanently to the micrometer head and tapping each micrometer screw into its slide.
To measure distances with the Fraunhofer micrometer, the position-circle is clamped at the true position-angle of the star, and the telescope is moved by its slow motions so that the component A of the star is bisected by the fixed wire; the other component B is then bisected by the web, which is moved by the graduated head S.
With the Cape micrometer a systematic difference has been found in the coincidence point for head above and head below amounting to o"-14.
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 I.
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.
Da and 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 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.
Parts of the micrometer screw in order to eliminate the effects of periodic error of the screw.
The electric lamp a gives illumination of the webs in a dark field, nearly in the manner described for the Cape transit circle micrometer; the intensity of illumination is regulated by a carbon-resistance controlled by the screw b.
MICROMETER (from Gr.