Vernier Calliper working principle:
Vernier scale is a visual aid, That allows the user to read measurements more precisely than any other instrument. Vernier scale indicates where the measurement lies in between two of the graduations on the main scale. See the following GIF to understanding how the vernier scale is moving over the main
http://home.iitk.ac.in/~vkjain/Lecture%204-Metrology-F-21-8-14.pdf
