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An inside micrometer is a device designed to precisely measure things such as the inner diameter of holes. Where a standard micrometer might be used to measure the outer diameter of a bolt, an inside micrometer could be used to measure the inner diameter of a nut. There are several designs for the inside micrometer, though they all perform this same basic function. All micrometers are designed to make very precise measurements, but the micrometers themselves can be either small or quite large. Each inside micrometer will have both a minimum and maximum diameter it is able to measure, so it's common to find inside micrometer sets.
There are several inside micrometer designs, though the most common are tubular- and caliper-type micrometers. A tubular inside micrometer consists of a cylindrical tube that may be placed in the inner diameter of a hole or ring to precisely determine the diameter, or between two parallel surfaces to measure the distance between them. Once the tubular micrometer is in place, there will usually be a way to extend the tube until it contacts both sides of the inner diameter of whatever is being measured. Caliper-type micrometers generally feature one stationary and one movable caliper mounted on a tube, with the ability to adjust the movable caliper out along the length of the tube to make a measurement.
Both standard and digital inside micrometers are available, with both relying on a screwing mechanism to display very accurate readings. Standard micrometers can be read by examining markings on the barrel and thimble of the micrometer, because very small distances are represented by greatly magnified rotational distances as the thimble is turned. Digital micrometers operate in the same way as standard micrometers, but they will have a digital readout on the barrel that negates the need to examine the barrel and thimble for scale and distance markings.
The unique feature of inside micrometers is their ability not only to measure small diameters and other distances very accurately, but also to measure very large diameters and distances accurately. To this end, inside micrometers may be found in many sizes. There are also sets that include a single measurement head and a number of tubes in varying lengths that can be attached to the head. This type of set can be used to measure a wide variety of inside diameters and various parallel surfaces without the need for multiple micrometers.