Micrometers measure the outside diameter of spherical and cylindrical objects by clamping the object between a stationary anvil and an adjustable spindle. Once you adjust the spindle to clamp the object that is being measured, lock the spindle in place to read the measurement.
After you clamp the object and lock the spindle in place, examine the sleeve of the micrometer to find the first unit of measurement. The sleeve of the micrometer is a stationary part. The markings on the sleeve are measured in quarter units. The thimble of the micrometer covers and uncovers the units on the sleeve as it rotates clockwise and counter-clockwise around the sleeve. The first unit of measurement is the last quarter uncovered by the thimble on the sleeve.
The thimble measures units to the thousandths and ten-thousandths decimal place. The thimble's markings align with the straight line that is perpendicular to the lines that mark each quarter on the spindle. The unit of measurement to record from the thimble is indicated by whichever unit aligns with the markings from the spindle. Each measurement obtained from a micrometer is written as a unit with three or four decimal places depending on the individual micrometer.