Hysteresis and related error mechanisms in the NIST watt balance experiment

The NIST watt balance experiment is being completely rebuilt after its 1998 determination of the Planck constant. That measurement yielded a result wi th an approximately 1 x 10(-7) relative standard uncertainty. Because the g oal of the new incarnation of the experiment is a ten-fold decrease in unce rtainty, it has been necessary to reexamine many sources of systematic erro r. Hysteresis effects account for a substantial portion of the projected un certainty budget. They arise from mechanical, magnetic, and thermal sources . The new experiment incorporates several improvements in the apparatus to address these issues, including stiffer components for transferring the mas s standard on and off the balance, better servo control of the balance, bet ter pivot materials, and the incorporation of erasing techniques into the m ass transfer servo system. We have carried out a series of tests of hystere sis sources on a separate system, and apply their results to the watt appar atus. The studies presented here suggest that our improvements can be expec ted to reduce hysteresis signals by at least a factor of 10-perhaps as much as a factor of 50-over the 1998 experiment.