Abstract
We have measured by interferometry the diameter of a fused-quartz hemispherical rotor to 200 nm, or an accuracy of 4 parts-per-million (ppm), at both room temperature and at liquid helium temperature. We placed the rotor in an optically contacted fused-quartz etalon, and used a continuous-wave dye laser to measure the lengths of the optical cavities. The laser frequency was referenced to Doppler-free absorption lines in molecular tellurium and molecular iodine. These measurements were done in order to determine the area of a superconducting ring used in a precision determination of h/me. Our results confirm earlier length measurements at room temperature performed by the National Bureau of Standards (NBS).
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