the least count of a mere scale is
Micrometer The accuracy of micrometers is checked by using them to measure gauge blocks,[16] 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).