What is an inside micrometer?
An inside micrometer is a tool used to measure the inner diameter of pipes and holes. For example, say you wanted an accurate measurement of a nut and bolt. You would use an outside micrometer to measure the diameter of the bolt. You would then grab an inside micrometer to measure the inside diameter of the nut.
Inside Micrometer Sets
Each inside micrometer has a certain range they are able to measure. This is why it is common to see these tools sold as a set. The Starrett model listed above is an example of an inside micrometer set. These allow you to take measurements at a greater range because you can swap measuring rods of different sizes.
Digital vs Standard
Like most micrometers, these are available in digital or standard readout.