Preliminary determination of G using the BIPM torsion strip balance

The BIPM is undertaking a determination of the Newtonian gravitational constant, G using a torsion balance with a thin, heavily loaded strip as the suspension element. This apparatus uses four 1.2 kg test masses and four 15.5 kg source masses to produce a gravitational torque of 2 × 10-8 N m. A preliminary determination of G has been made with a relative standard uncertainty of about 1.7 × 10-3, set primarily by the uncertainty in the measurement of the balance deflection by an autocollimator. The system will soon be used in a servo-controlled configuration, in which the gravitational torque will be balanced by an electrostatic torque applied between fixed thin cylindrical electrodes and the test masses. Operating in the servo mode should yield a significantly lower uncertainty by extending the autocollimator measurements over a larger range. A parallel effort will aim to lower the uncertainty of the unservoed measurement by using multiple reflections of the optical signal from the autocollimator.