Since its introduction in the 17th century, the gravitational constant, G,has been difficult to measure to great accuracy owing to the intrinsic weakness of the gravitational force. A new measurement of Gfrom researchers in Washington is the most accurate value yet, improving on the previous world best by a factor of 10.
The group modified the technique of Cavendish's 200-year-old torsion balance experiment with a thin, flat, rectangular plate hung vertically on a torsion fibre as the pendulum, and melon-sized stainless steel spheres as attractor masses, sitting on a turntable around the pendulum. As the turntable rotates, the gravitational forces exerted by the spheres try to twist the torsion fibre. However, a computer-controlled feedback mechanism adjusts the speed of the turntable, keeping the pendulum twisting to a minimum - an important factor for accuracy.
The acceleration of the turntable is recorded and gives the value of Gas 6.67390 ¥ 10-11 m3/kg/s2, with an uncertainty of 0.0014%. Combined with data from the Lageos satellite, more "best ever" results can be derived: the mass of the Earth is (5.97223 ± 0.00008) ¥ 1024 kg and the Sun's mass is (1.98843 ± 0.00003) ¥ 1030 kg. AIP