Word Finder - micrometer

definition

An SI/MKS unit of measure, the length of one millionth of a meter. Symbols: µm, um, rm.

The micrometer is at zero when the two edges are brought exactly together.

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.

In this micrometer the three slides moved by S, s, and s' are simple dovetails.

For the internal structural details of the micrometer the reader is referred to the article " Micrometer " in the 9th edition of the Encyclopaedia Britannica.

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.

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.

It became, in fact, essential to invent a " micrometer " for measuring the small angles which were thus for the first time rendered sensible.

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.

The Marquis Malvasia in his Ephemerides (Bologna, 1662) describes a micrometer of his own invention.

When equatorial mountings for telescopes became more general, no filar micrometer was considered complete which was not fitted with a position circle.'

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 represented in figs.

The eyepiece slides into the tube cd, which screws into the brass ring ef, through two openings in which the oblong frame, containing the micrometer slides, passes.

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.

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.

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.

The micrometer is clamped in FIG.

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.

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.

Huygens, in his Systema saturnium (1659), describes a micrometer with which he determined the apparent diameters of the principal planets.

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.

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 micrometer represented in fig.

The micrometer shown in fig.

He found that when the resonator was placed in certain positions with regard to the oscillator, small sparks were seen between the micrometer balls, and when the oscillator was placed at one end of a room having a sheet of zinc fixed against the wall at the other end, symmetrical positions could be found in the room at which, when the resonator was there placed, either no sparks or else very bright sparks occurred at the poles.

Among these subjects were the transit of Mercury, the Aurora Borealis, the figure of the earth, the observation of the fixed stars, the inequalities in terrestrial gravitation, the application of mathematics to the theory of the telescope, the limits of certainty in astronomical observations, the solid of greatest attraction, the cycloid, the logistic curve, the theory of comets, the tides, the law of continuity, the double refraction micrometer, various problems of spherical trigonometry, &c. In 1742 he was consulted, with other men of science, by the pope, Benedict XIV., as to the best means of securing the stability of the dome of St Peter's, Rome, in which a crack had been discovered.

The latter are in fact little microscopes carrying a vernier etched on glass, in lieu of a filar micrometer.

The illumination of the field is given by a lamp near the object glass, controlled by a switch near the micrometer.

Grubb's duplex micrometer is described in the 9th edition of the Encyclopaedia Britannica.

We have not had an opportunity of testing this, nor Grubb's more recent models; but, should it be found possible to produce such images satisfactorily, without distortion and with an apparatus convenient and rigid in form, such micrometers may possibly supersede the filar micrometer.

This involves a loss of accuracy because, with a spiderline micrometer, the accidental error of pointing is of the order of o I" of arc.

In the measuring machines in general use the field of view, as in the case of the glass-scale micrometer, is sufficiently large to include the image of the 5 mm.

The microscope or viewing telescope is fitted with a spider-line micrometer having two screws at right angles to each other, by means of which readings can be made first on one reseau-line, then on the star, and finally on the opposite reseau-line in both co-ordinates.

This form of micrometer is of course capable of giving results of high precision, but the drawback is that the process involves a minimum of six pointings and the entering of six screw-head readings in order to measure the two co-ordinates of the star.

The object glass of the micrometer-microscope is placed midway between the plane of the photographic plate and the plane of the micrometer webs.

The micrometer is provided with a " fixed square " 5 mm.

Two other screws, o, p, the heads of which are not graduated, give motions to the whole micrometer box through t 1 mm.

All the essential parts of the micrometer, including the slides, micrometer box, tube, etc., are of steel or cast-iron, so that changes of temperature do not affect the adjustments.

The micrometer readings for coincidence of the movable webs with the webs of the fixed square shall be exactly 0 000R and io-000R.

By means of the quick rack motions A and B move the plate so as to bring the reseau-square into the centre of the field of the micrometer; then, by means of the screw heads o, p, perfect the coincidence of the " fixed square " of webs, with the image of the reseau-square.

Its fundamental principle is that, by a combination of glass scales with a micrometer screw, " the chief part of the distance to be measured is read off on the scale; the fractional part of the scalespace is not estimated but measured by the screw."

An important modern application of the micrometer, which is not dealt with in the article Transit Circle, is that which is now called " the travelling wire micrometer."

It should be mentioned that an essential feature of the travelling wire micrometer is that the eyepiece as well as the wire shall be moved by the micrometer-screw.

The method originally used by Huggins, who first conceived and proved the possibility of measuring stellar velocities in the line of sight, was to measure with a filar micrometer the displacement of some well-known line in a stellar spectrum relative to the corresponding line of a terrestrial spectrum.

The capillary tube can be raised or lowered at will by running a magnet outside the tube, and the heights of the columns are measured by a cathetometer or micrometer microscope.

The position of the middle of the bright band representative of a mathematical line can be fixed with a spider-line micrometer within a small fraction of the width of the band, just as the accuracy of astronomical observations far transcends the separating power of the instrument.