These micrometers come with a calibration certificate traceable to NIST that states they've passed a test for accuracy. They do not require batteries; a mechanical analog counter displays measurement readings. All have a friction thimble with a constant-force mechanism, so that once the micrometer’s spindle contacts the surface of the measured object, it will stop tightening. This ensures repeatable measurements and prevents over-tightening. Once the spindle has stopped tightening, the thimble will continue turning with resistance. A speeder knob reduces the time it takes to open and close the micrometer.