Measuring the gravitational constant with a cryogenic torsion pendulum

Concluding results from an extensive program to measure the gravitational constant G will be presented. Data were obtained from y2000 to 2006 using a torsion pendulum in the 'dynamic' ("time-of-swing") mode, operating at temperatures ~2.7 K. Six torsion fibers of three types were used with torsional oscillation amplitudes between 0.3 and 7.4 radians: BeCu as-drawn, BeCu heat treated, and Al5056 as-drawn. A metrology scale factor is hidden until data and metrology analysis are complete, to minimize danger of experimenter bias. Some evidence for both amplitude and fiber material dependence is found which is expected to limit our uncertainty to about 50 ppm. A final G value will be reported at CPEM2008.