The inside micrometer is a measuring tool used to measure the inside of holes or bores. You might use an inside micrometer to measure the wear in an engine cylinder. Measuring rods of different lengths come with the micrometer. You can do different ranges of measurement by assembling different rods in the micrometer head.
Inside micrometers are made with both inch and metric system scales. The inside micrometer is read like that of the outside micrometer. More practice is required to get an accurate measurement with an inside micrometer. You must hold it perfectly straight at right angles to the centerline of the hole being measured. Then move one end back and forth slightly to get the maximum reading. Always take two or three additional reading as an accuracy check.