- An instrument for measuring accurately dimensions or separations. Its basis is that when a screw is turned once, it advances or retreats a distance equal to its pitch. The hairlines in telescope and microscope eyepieces are adjusted by means of a precision micrometer screw to measure separations.
The micrometer caliper has a G-shaped frame on whose "leg" is a scale; inside the leg runs a screw (of pitch usually 0.5 mm) attached to a thimble, calibrated for fractions of a turn, which runs over the scale. An object is placed between the screw's spindle and an anvil at the far side of the G's opening, and the screw turned until the object is just held.For greater accuracy (of the order of 1μm) a vernier scalemay be used. - A unit of length, equal to one millionth of a meter (μm). (British spelling: micrometre.) Known in the CGS system of units as the micron.
The micrometer is a precision measuring instrument, used by engineers. Each revolution of the rachet moves the spindle face 0.5mm towards the anvil face. The object to be measured is placed between the anvil face and the spindle face. The rachet is turned clockwise until the object is ‘trapped’ between these two surfaces and the rachet makes a ‘clicking’ noise. This means that the rachet cannot be tightened any more and the measurement can be read.
EXAMPLE MEASURE READINGS
Using the first example seen below:
1. Read the scale on the sleeve. The example clearly shows12 mm divisions.
2. Still reading the scale on the sleeve, a further ½ mm (0.5) measurement can be seen on the bottom half of the scale. The measurement now reads 12.5mm.
3. Finally, the thimble scale shows 16 full divisions (these are hundredths of a mm).
The final measurement is 12.5mm + 0.16mm = 12.66
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