Precise determination of h/m/sub e/ using a rotating, superconducting ring

A precise determination of h/m/sub e/ (Planck's constant divided by the free electron mass) using a rotating, superconducting ring is presented. The measurement is based on two macroscopic quantum properties of superconductors: the quantization of the magnetic flux inside a superconducting ring and the London moment, a magnetic moment generated by any rotating superconductor and proportional to the spin speed. When these two magnetic fluxes are balanced against each other (a flux null), h/m/sub e/ can be found by measuring the spin speed at that point and the area bounded by the ring. The superconducting ring used in these experiments was thin film niobium (20-μm-wide by 400-nm-thick) deposited by electron-beam evaporation around the equator of a 5-cm-diameter, fused-quartz hemisphere. A novel photolithographic, lift-off technique was used to define its width and location. The cross-sectional areas of this ring were determined to an accuracy of 8 ppm. The hemisphere also acted as the rotor for an all-quartz, cryogenic, helium-gas bearing. Simultaneous measurements of the rotor's spin speed and the quantized magnetic flux inside the ring generated the information necessary to calculate the value of h/m/sub e/. This work has determined h/m/sub e/ directly to an accuracy of 100 parts per million (ppm), a higher level of precision than previously reported (400 ppm). Recent theoretical calculations have predicted a relativistic mass increase of 150 ppm for electrons in spinning, superconducting niobium, but the uncertainty in the final results prevented the clear identification of this effect