Micrometer is a simple device to measure very small lengths easily. The movable cylindrical scale is calibrated into 50 units. If it is rotated two full rounds, it moves 1mm along the fixed scale.
100 Vernier units = 1mm
1 Vernier unit = 0.01mm
Therefore, the Vernier scale lets us measure a length up to 0.01mm. The following animation describes the working of the device.
This is how a micrometer is used in practice.