Mechanical internal micrometers, or sometimes referred to as analogue internal micrometers are used to measure internal dimensions. Inside micrometers are suitable for measuring internal bore diameters and other internal dimensions like pocket features. Inside micrometers come in many forms. Some types are similar in design to the internal jaws of a pair of calipers. More complex and exact types, like ones specialised for bore dimensions, use 3-points of contact for more reliable measurements.
Internal micrometers are typically more precise than calipers and can offer precision resolutions as little as one micron on ultra-precision models. Some of the models are supplied with an ultra-precision ring-gauge which is used for the inside micrometer calibration procedure. We have written an helpful article on how to use inside micrometer and you can find the link below.