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Photograph of Fischer Transverse Invar Beam Comparator
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Photograph of Fischer Transverse Invar Beam Comparator

 ca. 1904-1921  NIST Historical Collection
The Fischer Transverse Invar Beam Comparator was designed by Louis Fischer for making comparisons between line-standards (a line standard is a metal measuring rod where the standard is the distance between two marks on its surface). Its design evolved from that of a comparator that was temporarily installed in the subbasement of the Butler Building so that Fischer could make a comparison of National Prototype Meters No. 21 and No. 27. This was immediately before Fischer accompanied No. 27 to Sevres in 1903 for a comparison at the International Bureau of Weights and Measures (BIPM). The later comparator consisted of a carriage that moved transversely to a low-expansion invar beam (the earlier comparator's beam was iron) on which were clamped two microscopes. Mounted on the carriage was a brass and wood box that held the two length standards at equal temperatures. The microscopes moved from opposite ends of the beam to focus on lines scribed on the surfaces of the length standards with turns of carefully calibrated micrometer screws. Only standards of the same nominal length could be compared on Fischer's instrument.

Follow a path that traces the historical development of length comparators.


Schematic Drawing of Historical Comparator