<P> On most micrometers, a small pin spanner is used to turn the sleeve relative to the barrel, so that its zero line is repositioned relative to the markings on the thimble . There is usually a small hole in the sleeve to accept the spanner's pin . This calibration procedure will cancel a zero error: the problem that the micrometer reads nonzero when its jaws are closed . </P> <P> A standard one - inch micrometer has readout divisions of . 001 inch and a rated accuracy of + / - . 0001 inch ("one tenth", in machinist parlance). Both the measuring instrument and the object being measured should be at room temperature for an accurate measurement; dirt, abuse, and low operator skill are the main sources of error . </P> <P> The accuracy of micrometers is checked by using them to measure gauge blocks, rods, or similar standards whose lengths are precisely and accurately known . If the gauge block is known to be 0.7500" ± . 00005" ("seven - fifty plus or minus fifty millionths", that is, "seven hundred fifty thou plus or minus half a tenth"), then the micrometer should measure it as 0.7500 ". If the micrometer measures 0.7503", then it is out of calibration . Cleanliness and low (but consistent) torque are especially important when calibrating--each tenth (that is, ten - thousandth of an inch), or hundredth of a millimeter, "counts"; each is important . A mere spec of dirt, or a mere bit too much squeeze, obscure the truth of whether the instrument is able to read correctly . The solution is simply conscientiousness--cleaning, patience, due care and attention, and repeated measurements (good repeatability assures the calibrator that his / her technique is working correctly). </P> <P> Calibration typically checks the error at 3 to 5 points along the range . Only one can be adjusted to zero . If the micrometer is in good condition, then they are all so near to zero that the instrument seems to read essentially "- on" all along its range; no noticeable error is seen at any locale . In contrast, on a worn - out micrometer (or one that was poorly made to begin with), one can "chase the error up and down the range", that is, move it up or down to any of various locales along the range, by adjusting the sleeve, but one cannot eliminate it from all locales at once . </P>

The least count of a mere scale is