Preliminary determination of G using the BIPM torsion strip balance

The BIPM is undertaking a determination of the Newtonian gravitational cons tant, G using a torsion balance with a thin, heavily loaded strip as the su spension element. This apparatus uses four 1.2 kg test masses and four 15.5 kg source masses to produce a gravitational torque of 2 x 10(-8) N m. A pr eliminary determination of G has been made with a relative standard uncerta inty of about 1.7 x 10(-3), set primarily by the uncertainty in the measure ment of the balance deflection by an autocollimator. The system will soon b e used in a servo-controlled configuration, in which the gravitational torq ue will be balanced by an electrostatic torque applied between fixed thin c ylindrical electrodes and the test masses. Operating in the servo mode shou ld yield a significantly lower uncertainty by extending the autocollimator measurements over a larger range. A parallel effort will aim to lower the u ncertainty of the unservoed measurement by using multiple reflections of th e optical signal from the autocollimator.