This instrument was built at the Office of Weights and Measures (OWM) from Joseph Saxton's design. It was probably used first for comparing the yard end-standard that was part of the standard yard apparatus distributed by OWM to the states and custom-houses and, after 1866, for comparing meter end-standards. Saxton's comparator could measure differences of 1 micrometer (1 millionth of a meter). It used a sensitive indicating mechanism, which was based on the rotation of a small mirror, to detect differences between the standard and the measuring rod being calibrated. The mirror was mounted on a shaft which was coupled to a spring-loaded slide that rested against the end of the standard. Very small differences in rod length caused the mirror to rotate a small amount. These small rotations of the mirror were greatly amplified by sighting through a telescope where the image of a graduated scale was reflected into the telescope by the mirror. The instrument was used by first adjusting the rotating mirror so that the scale read zero when the standard was in place and then reading the difference, in micrometers, when the rod to be measured was replaced by the standard.

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