Initial Results from a New Measurement of the Newtonian Gravitational Constant

Abstract

The Newtonian gravitational constant, G, remains the least well-known of the fundamental constants of physics in spite of several recent and current efforts. One of these is underway at the National Bureau of Standards in cooperation with people from the University of Virginia. Our experiment is of the type originated by Beams et al.^l In such an experiment gravitational force is balanced against the force of inertial reaction in an accelerated rotating reference frame. It is this acceleration which is measured to yield a value for G. In its present incarnation, we feel that the measurement has a potential accuracy of about 10 ppm. The principle of the measurement is recalled in Fig. 1. The small mass system (first a cylindrical bob and later a dumbbell), the large masses, the auto-collimator, the fiber which supports the small mass and an encoder disc are mounted on the rotating table. The auto-collimator senses the tendency of the small mass system to rotate toward alignment with a line joining centers of the large masses in response to the gravitational attraction. The torquer accelerates the rotating table so as to cancel the gravitational attraction, thus keeping the small mass stationary in the rotating frame. As seen in a laboratory-based frame the table accelerates uniformly over some (tolerable) number of revolutions. Clock readings at various table positions indicated by the disc encoder are the raw data of the measurement. The entire apparatus is enclosed in an acoustic chamber about 2.5 meters on an edge and mounted on a reinforced concrete slab of about 5000 kilograms.