With their focus on marketplace equity, state metrology laboratories are generally less concerned with the accuracy of their length standards than with the accuracy of their standards of mass and capacity. However, in machine shops, the lengths of short workpieces are measured by comparing them with a stack of accurately ground and polished end standards known as gage blocks. Over 100 000 length combinations are obtainable from a set of 81 blocks. The modern abundance of precision machines at low cost could not have been achieved without production methods utilizing interchangeable parts, and this, in turn, could not have been achieved without modern precision gaging methods using gage blocks. National Institute of Standards and Technology (NIST) master gage blocks, both English and metric, are calibrated in interferometers using an iodine stabilized helium-neon laser. These interferometer systems are directly traceable to the definition of the meter based on the speed of light.
Between 5000 and 6000 gage blocks are sent to NIST each year by industry, foreign industrial customers, and other federal government agencies for calibration or recalibration. Gage blocks belonging to NIST customers are compared to NIST master gage blocks using a probe-type instrument that touches the gaging surface of each block, alternating between the master and the customer's block. The difference between the master and the customer's block is sensed with a linear-variable-differential-transformer (LVDT) transducer attached to the probe. This comparator has a resolution of 1 nanometer (about 75 000 nanometers equal the thickness of a human hair.) To calibrate a set of 81 customer blocks, a NIST master set of 81 is required. The blocks calibrated at NIST often become the "masters" in the customer's own metrology laboratories.
Follow a path that traces the historical development of length comparators.
