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A caliper is an instrument used for measuring linear dimensions that are not easily measured by devices such as meter sticks or rulers. Two examples of such measurements include the outer dimensions of a pipe or the internal diameter of a glass tube.

Although many kinds of calipers exist, they are all designed on a common principle: two legs are hinged at one end to allow movement of the free ends of the legs both towards and away from each other. A caliper looks like a common pair of tweezers. The distance between the free ends of the two legs is the linear dimension measured by the caliper.

The crescent-shaped legs of an outside caliper are curved inward toward each other. When the ends of the caliper are placed on the outside of some object, the distance between the ends of the legs can be read on a scale (usually incised on the pivot end of the caliper), giving the outside diameter of the object. The legs of an inside caliper, on the other hand, are curved away from each other, like an hourglass. To find an interior diameter, the caliper is placed inside an object and opened. Again, the distance between the ends of the caliper can be read on a scale.

The micrometer is one of the most common devices using the caliper principle. The micrometer consists of a metal handle around which a movable cylinder called the thimble is attached. As the thimble is rotated, a spindle connected to the handle moves toward or away from a fixed anvil. The dimensions of an object can be measured by placing the object between the anvil and the spindle and slowly rotating the thimble. When the object is in firm contact with the anvil on one side and the spindle on the other, its linear dimensions can be read on the scale located on the micrometer handle.

The micrometer caliper can provide precise measurements relatively easily. A turn of the thimble advances the spindle only a small distance. A single rotation of the thimble in most micrometers advances the spindle a distance of 0.025 in (0.064 cm). The thimble itself is divided into 25 segments, which enables the micrometer to measure distances as small as 0.001 in (0.0025 cm). The addition of a vernier scale to the micrometer can further improve the precision of a measurement by a factor of 10.

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