A measurement of G with a cryogenic torsion pendulum

Abstract

A measurement of Newton's gravitational constant G has been made with a cryogenic torsion pendulum operating below 4 K in a dynamic mode in which G is determined from the change in torsional period when a field source mass is moved between two orientations. The source mass was a pair of copper rings that produced an extremely uniform gravitational field gradient, whereas the pendulum was a thin fused silica plate, a combination that minimized the measurement's sensitivity to error in pendulum placement. The measurement was made using an as-drawn CuBe torsion fibre, a heat-treated CuBe fibre, and an as-drawn Al5056 fibre. The pendulum operated with a set of different large torsional amplitudes. The three fibres yielded high Q -values: 82 000, 120 000 and 164 000, minimizing experimental bias from fibre anelasticity. G -values found with the three fibres are, respectively: {6.67435(10),6.67408(15),6.67455(13)}×10 −11  m 3  kg −1  s −2 , with corresponding uncertainties 14, 22 and 20 ppm. Relative to the CODATA2010 G -value, these are higher by 77, 37 and 107 ppm, respectively. The unweighted average of the three G -values, with the unweighted average of their uncertainties, is 6.67433(13)×10 −11  m 3  kg −1  s −2  (19 ppm).