Reading Micrometers - 5: The Vernier Scale
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WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor.
A Vernier scale is the small scale placed next to the thimble scale (above the reading line). It is named after its inventor, Pierre Vernier. This scale allows you to make measurements to a precision of 1/10th of a thimble division. Since each of the thimble divisions signify .001 of an inch then that means that the Vernier scale allows you to read "between" the thimble marks to an accuracy of .0001 of an inch.
For very small movements of the thimble a single line on the vernier scale will align with one, and only one, line on the thimble. It does not matter which line on the thimble is the matching line and the thimble numbers can be totally disregard Only the line on the vernier scale is important.

There are ten positions where the alignment takes place and each position represents .0001 inch. Just remember that the vernier scale reads the measurement that lies between thimble divisions. How many divisions are there between .100 and .101? The answer is nine. They are .1001, .1002, .1003, .1004, .1005, .1006, .1007, .1008, and .1009
Notice in the animation below that the vernier scale matches with a line on thimble every .0001 inch.

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