Session J9 - Experimental Gravity and Gravitational Radiation.
FOCUS session, Sunday afternoon, April 29
Room 2, Renaissance Hotel
We have measured the gravitational constant, G, with a relative uncertainty of \pm14 ppm. Our new method was designed to minimize systematic uncertainties. A torsion balance was installed on a slowly rotating turntable located between a set of attractor spheres. The rotation rate of the turntable was varied so that the gravitational torque did not twist the torsion fiber and therefore complex properties of the torsion fiber, which had led to a bias in other measurements, were eliminated. The amplitude of the sinusoidal turntable acceleration was measured and is proportional to G. The geometry of our pendulum was a thin flat vertical plate, which made our measurement practically independent of the pendulum mass distribution. Traditionally the pendulum metrology had been one of the largest uncertainties. The attractor masses were located on a second, co-axial, turntable that was rotated with a constant angular velocity difference relative to the pendulum turntable. This allowed us to discriminate against attractions to other objects in the lab. Choosing a high difference velocity and therefore a high signal frequency reduced 1/f-noise.
Our value for Newton's constant is G = (6.674215\pm 0.000092) 10^-11 m^3kg^-1s^-2. When combining our value with results from the LAGEOS satellites the Earth's mass is: M_earth = (5.972245\pm 0.000082) 10^24 kg.
Supported with NIST precision measurement grant # 60NANB7D0053 and NSF grant # PHY-9602494.