Micrometer

[Sarah]

An instrument, used with a telescope or microscope, formeasuring minute distances, or the apparent diameters of objectswhich subtend minute angles. The measurement given directly is thatof the image of the object formed at the focus of the object glass.Circular, or Ring, micrometer, a metallic ring fixed in the focus ofthe object glass of a telescope, and used to determine differences ofright ascension and declination between stars by observations of thetimes at which the stars cross the inner or outer periphery of thering.-- Double image micrometer, a micrometer in which two images of anobject are formed in the field, usually by the two halves of abisected lens which are movable along their line of section by ascrew, and distances are determined by the number of screwrevolutions necessary to bring the points to be measured into opticalcoincidence. When the two images are formed by a bisected objectsglass, it is called a divided-object-glass micrometer, and when theinstrument is large and equatorially mounted, it is known as aheliometer.-- Double refraction micrometer, a species of double imagemicrometer, in which the two images are formed by the doublerefraction of rock crystal.-- Filar, or Bifilar, micrometer. See under Bifilar.-- Micrometer caliper or gauge (Mech.), a caliper or gauge with amicrometer screw, for measuring dimensions with great accuracy.-- Micrometer head, the head of a micrometer screw.-- Micrometer microscope, a compound microscope combined with afilar micrometer, used chiefly for reading and subdividing thedivisions of large astronomical and geodetical instruments.-- Micrometer screw, a screw with a graduated head used in someforms of micrometers.-- Position micrometer. See under Position.-- Scale, or Linear, micrometer, a minute and very delicatelygraduated scale of equal parts used in the field of a telescope ormicroscope, for measuring distances by direct comparison.

[Rihanna]

a device used for making very exact measurements or for measuring very small things