Fig. 1: Micrometer.
Micrometers are used to make precise measurements of small objects. An example is shown in figure 1.
To open and close the jaws of the micrometer you should use the friction screw on the end as this prevents over-tightening. Some micrometers have a locking lever which you should put in place once the jaws are closed on the object you are measuring.
Fig. 2: Micrometer example.
The micrometer has two scales, a coarse scale on the rod and a fine scale on the thimble. One rotation of the thimble moves the jaws in or out by 0.5mm. To take a reading, we first read off the coarse scale which will give us a reading to the nearest half-millimeter. In the example in figure 2 it is 6.5mm. Then to read the fine scale on the thimble, look at where the horizontal line on the sleeve is pointing. In figure 2 it is at 7, so the reading is 6.5mm + 0.07mm = 6.57mm.
To use a micrometer yourself, check out our interactive micrometer experiment on our ISE page.