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What Is a Micrometer?

The precision instrument used to measure small distances and angles is called a micrometer. It comprises a spindle attached to an accurately machined screw, which is rotated by a thimble or ratchet knob until the object to be measured is in contact with the spindle and an opposing, fixed anvil. The ratchet 'slips' after a predetermined pressure has been applied, ensuring consistent, accurate gauging.

A micrometer is widely used in engineering to measure the diameter of round objects or the exact thickness of flat pieces.

In metric micrometers, the pitch on the screw is a uniform one millimeter. When the crew is rotated, the thimble, marked in equal divisions, is turned. Once the object being measured is firmly held between the spindle and anvil, the operator is able to read the scales marked on the thimble and shaft to the distance between the flat ends of the anvil and spindle. The scale on the shaft of the spindle is a vernier scale, which enables the thimble scale to be further subdivided by a factor of 10.

The micrometer was invented in about 1638 by William Gascoigne, an English astronomer, as an improvement on the vernier scale. He used it to measure the diameter of the Sun, Moon and planets. This principle is now the essential one on which instruments for astronomers, engineers and machinists work. There are many uses for the instrument. It can be used as a comparator for measuring polarized light and to adjust the movable mirror in an interferometer.

It is an essential mechanism in a spherometer, measuring either convex or concave curvatures and is particularly useful for lens grinding. Civil engineers use an attachment at the eyepiece of theodolites, similar to those employed on telescopes and microscopes, to measure minute astronomic and microscopic phenomena. However, the most common use of the micrometer is as a machinist's caliper.

Micrometers are produced in graduated sizes, the most common measuring from zero to 25 mm, the next size measuring 25 to 50 mm and so on. As a measuring device, it is used by motor mechanics, instrument technicians, toolmakers and many others. In the manufacture of many durable consumer goods, there may be hundreds of inspections and micrometers are invaluable precision tools throughout these many processes.

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