What are the usages of micrometers?

A micrometer is a guage measuring device used to measure small lengths with an accuracy of 1\100 mm

There are 2 things that can be called a micrometer: 1/ A device for measuring short distances accurately. Typically to better than 1 thousands of an inch (thou) or 0.01mm. They look a bit like a...

custom (an accepted or habitual practice)

On is a preposition, indicating position (sometimes metaphorically).

'Which' is used when talking about one of a group of things known or implied by the speaker and listener. 'What' can refer to more unknown things, e.g. 1 Which is best, coke or lemonade? 2 What is...