Several precautions are necessary to follow when you use the micrometers to take measurements. In general, two turns of the thimble are required to advance one division along the barrel scale. Thus you will need to notice that whether one of these turns already has been made when reading the circular scale. For example, a micrometer with mm markings on the barrel scale and 50 divisions on the circular scale, but which requires two turns to advance one unit along the main scale has a least count of 1 mm / ( 2 x 50) = .01 mm. A reading of "25" on the circular scale of such a micrometer therefore could mean either 0.25 mm or 0.75 mm (0.50 + 0.25). The operator must be able to discern these readings.
Usually, the threads of most micrometer screws are very delicate and easily distorted by forcing the jaws against the object being measured. Even without distortion, different readings will result from different applied force of the micrometer jaws. Because most micrometers have a ratchet on the end which will slip when the proper force is applied. Use this ratchet to insure constant pressure when taking measurements.
Secondly, to check for zero error and to make suitable corrections. Most micrometers can be mechanically adjusted to eliminate zero error, but we will not make these adjustments. Simply correct for the zero error.