Micrometer

“ The micrometer is so set that when a gage, G, of exact size, is placed between the measuring points, the long arm of the indicator stands at the 0 mark. If the pieces being calipered vary in the least from the standard size it will be readily noted by the movement of the pointer. Hard rubber shapes turned from rough casting often vary from 0.003 to 0.005 inch after having passed the inspector's test with an ordinary micrometer. With this attachment the inspector's helper can detect very minute variations from the limit size. ”

“ Suppose A, B, C three micrometer microscopes very firmly supported at intervals of 4 metres with their axes vertical, and aligned in the plane of the base line by means of a transit instrument, their micrometer screws being in the line of measurement. The measuring bar is brought under say A and B, and those micrometers read; the bar is then shifted and brought under B and C. By repetition of this process, the reading of a micrometer indicating the end of each position of the bar, the measurement is made.
Quite similar apparatus (among others) has been employed by the French and Germans. ”

“ In delicate researches two divisions of the scale should always be read, not merely for increased accuracy but to obtain the corrections for “run” from the observations themselves.
Repsolds employ for the micrometers of their reading microscopes the form of construction shown in fig. 9, omitting, of course, the motion of the whole micrometer box given by the screw s for those cases in which the axis of the micrometer is supposed to remain constant in position, as, for example, in the case of the reading microscopes of transit circles (see Transit Circle) . ”

“ 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. ”

“ The micrometer should not be fitted into too deep an eye-piece, as it is essential to preserve good definition. A middle-power Kellner or Huyghenian is frequently employed; at all events, use the eye-piece of lower power rather than impair the image.
The eye-lens above the micrometer should not be of shorter focus than three-quarters of an inch, even with high-power objectives.
The Ramsden Eye-piece.—The cobweb micrometer is the most efficient piece of apparatus yet brought into use for measuring the magnified image. ”

“ 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. In both cases two eyepieces are employed, one to view each separate web. One drawback to this form of instrument is that the two webs cannot be viewed simultaneously, and therefore the observer must rely on the steadiness of rate of the clockwork and uniformity in the conditions of refraction whilst the eye is moved from one eyepiece to the other. ”

“ The cathetometer telescope has a lens of such a focus that 1 mm. on the wood section becomes 0.25 mm. in the focus. The micrometer has a screw-thread with a pitch of 0.25 mm., so that one revolution of the micrometer head moves the thread through exactly 1 nun. as seen on the wood. The individual measures of rings are made on the micrometer screw by reading the graduation of the head to revolutions and hundredths, giving directly millimeters and hundredths. ”

“ The screw micrometer is not so exact on a yielding material like paper as is the spring micrometer, which gives the reading on a dial in thousandths of an inch. Machines are obtainable in pocket or stand form, giving measurements as close as 1/2000 of an inch. ”

“ The micrometer may be used in combination with a 4-, 6-, 9-, or 12-inch scale. It can be adjusted on standard plugs, or one can make a set of gages up to 12 inches, out of 3/16-inch round tool steel wire, and use these for setting. In mounting the micrometer, before cutting it apart, mill the shoulders shown at A, and in milling the bottom pieces B, use a piece of machine steel long enough for both, cutting the piece in half after milling the slots. In this way one obtains perfect alignment. In a shop where a set of large micrometers is not kept, this arrangement is very useful. [14] ”

“ The eyepiece micrometer allows its errors to be diminished, if one measures at different points and then fixes a mean value. The dead motion of a micrometer screw is best avoided by working the screw always from one and the same side. The thickness of the cross wire may also occasion a fault. For this reason there is sometimes employed two very narrow threads lying beside one another, and which limit the image as nearly as possible.
The Testing of the Microscope
The excellence of a microscope objective depends on its definition and its resolving power. ”

“ Notice how many divisions of the micrometer in the body are seen in one division of the micrometer under the magnifier, which again must be multiplied by the power of the eye glass. Example: Ten divisions of the micrometer in the body are contained in one division under the magnifier; so far the power is increased ten times: now, if the eye glass be one inch focus, such glass will of itself magnify about eight times in length, which, with the ten times magnified before, will be eight times ten, or 80 times in length, 6400 superficies, and 512000 cube. ”

“ A micrometer consists of a short tube, across the opening of which are stretched two parallel wires; these being intersected at right angles by a third. The wires are moved to or from each other by delicately constructed screws, to which they are attached. ”

“ The field of the telescope may be provided with a graduated scale, or a metallic ring, or a diaphragm having parallel and intersecting spider-lines or fine wires. The micrometer with a graduated scale is used for measuring distances by direct comparison. ”

“ The micrometer screw itself has a travel of one inch, like any standard micrometer. A locknut is used to hold the screw in any desired position. A thumb screw at the end of the barrel bears against the end plug, and zero marks are provided to bring the screw against the plug with the same degree of pressure at each setting. When the head is clamped by means of the locking nut, it is as rigid as though it were solid with the barrel, and the faces of the measuring points are thus always parallel.
Fig. 25. Special Micrometer for Large Dimensions
Combination Micrometer ”

“ In this way the scale can be viewed by a microscope of much higher magnifying power than can be employed for the photographed spectrum. Indeed, if the scale were subdivided to 1/10 mm. the power employed might only be limited by the sharpness of the division-lines. But for refined work this would imply the investigation of too many divisions of the scale; it is therefore more usual to divide the scale into single millimetres or half-millimetres and to provide a micrometer which subdivides the millimetre into 1000 or, by estimation, into 10,000 parts. ”

“ A micrometer is an instrument attached to a telescope, in order to measure small spaces in the heavens, such as the spaces between two stars, and the diameters of the sun, moon and planets—and by the help of which the apparent magnitude of all objects viewed through telescopes may be measured with great exactness. ”

“ 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. The lamp c illuminates the drum-head and also, by reflection, the portions of the position-circle which come under the microscopes d and e. The head f is a switch which enables the observer to illuminate lamp a or c at pleasure. These lamps, although shown in the figure, are in reality covered so as not to shine upon the observer’s eye. ”

“ If the top lense of the eye-piece be unscrewed, a diaphragm will be found on which the eye-piece micrometer will easily rest. Screw on the top lense again and, generally, the eye-piece will be ready for use. If the micrometer is not properly in focus after a few trials, it may easily be made right. In order, then, to measure the diameter of a single fibre of Sea Islands cotton, fit in the quarter inch objective and place the [Pg 70] stage micrometer in position on the stage. ”

“ 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, [8] 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. It will be sufficient to describe those forms with which the most important work has been done, or which have survived the tests of time and experience. ”

“ It will therefore be seen that objects as small as a thousandth of a millimeter can be accurately measured by the ocular micrometer.
In making microscopic measurements it is only necessary [21] to find how many lines of the ocular scale are covered by the object. The number of lines multiplied by the equivalent of each line will be the size of the object in microns, or micromillimeters. ”

“ Fig. 29 shows a very simple and light five-inch micrometer that can be quickly set to exact position from one to five inches. The round beam is graduated by a series of angular grooves, 1 inch apart, which are of such a form and depth that the clamping fingers at the end of part A spring in, allowing one inch adjustment of the beam to be quickly and positively made. ”

“ 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 scale space is not estimated but measured by the screw.” Hinks claims that thus never more than one- or two-tenths of a revolution of the screw need be used in making the measure, and little time is lost in running the screw backwards and forwards. ”

“ Beck’s Star Microscope is in every sense a students’ or class-room instrument. It is firm and well made, with joint for inclination, large square stage, sliding coarse adjustment and fine adjustment by micrometer screw, draw-tube, iris diaphragm, double mirror on swinging crank arm, A or B eye-piece, a one-inch and quarter-inch objective, the magnifying power of which ranges from 38·5 to 183. ”