Least Count of an instrument is the smallest measurement that can be taken using an instrument. In other words, it is the smallest change an instrument can measure.
The spindle of an ordinary metric micrometer has 2 threads per millimetre, and thus one complete revolution moves the spindle through a distance of 0.5 millimetre. The longitudinal line on the sleeve is graduated with 1 millimetre divisions and 0.5 millimetre subdivisions.
Hence Metric outside micrometer has a threaded spindle with a pitch of 0.5 mm
Zero error in micrometer occurs when zero on the thimble and datum line on sleeve do not coincide when measuring faces are in contact.
The lock nut is simply a quarter turn nut that locks the spindle from turning. The sleeve contains the scale which is used to determine the measurement and the internal threads which allow the spindle to move.
The working principle of the Micrometer is based on screw and nut. (Rotational moment to linear moment) As we know as the screw rotates one revolution on the screw, then the screw moves linearly by one pitch distance