A new method for measuring the gravitational constant G is described. Preliminary measurements give G= (6.674 ±0.012) xio"" 11 N m 2 /kg 2 where the 0.012 represents 3 standard deviations. Furthermore there is reason to believe that with certain modifications of the apparatus and use of improved metrology techniques an increase in precision of at least one and probably two orders of magnitude will be obtained.
Several investigators have searched without success for axial potential gradients produced in metal spinning rotors. Lodge 1 and Nichols 2 looked for the effect in metal discs spinning in air at atmospheric pressure, but the comparatively large and variable electromotive forces introduced by the rubbing electrical contacts on the axis and on the periphery made their results inconclusive. More recently Freedman 3 and his associates also were unable to find the phenomenon with more reliable equipment. In this paper, experiments are described in which a small radial potential difference is observed across a rapidly spinning Duraluminum (ST14) rotor surrounded by air at pressures between 10~5 and 10"" 6 Torr. Robl, 4 Schiff and Barnhill, 5 Dessler, Michel, Rorschach, and Trammell, 6 and Herring 7 have shown theoretically that a gravitationally induced electrical field should exist outside of a vertical conductor. In a most ingenious experiment, Witteborn and Fairbank 8 have recorded a vertical field of 5.6x 10"" 11 V/m inside of a copper cylinder, a result which is in agreement with the theory of Schiff and Barnhill. Since centrifugal fields can be made very much largen than the earth's gravitational field, the effect should be correspondingly larger in a rapidly spinning rotor. 9 Figure 1(a) shows a schematic diagram of the experiment. The rotor R was spun inside the evacuated metal chamber V by an air-supported, air-driven turbine T situated below V. The thin flexible shaft S which connects the turbine to the rotor passes through the electrically insulated vacuum-tight oil glands G x and G 2 . This scheme for supporting and driving high-speed rotors in a vacuum has been previously described; so the mechanical details will not be given. 10 Electrical connection with the rotor was made through a water-cooled liquid-mercury contact with the shaft at M. The rotor, the vacuum chamber, bearings, turbine, etc. were all nonferromagnetic. This simplified the compensation of the earth's magnetic field at the rotor by a large Helmholtz coil which surrounded the apparatus. The turbine drive was so designed that the direction of rotation could be reversed. The vacuum chamber and the electrical and oil shields were made of metal and were grounded. Figures 1(a) and 1(b) show a schematic cross section (not to scale) and top view of the rotor which is machined in the form of a cross. The rotors were 15 cm in diameter and 2 cm thick, and the cross arms were 2 cm wide. The rotating parts were electrically insulated from the stationary parts by vacuum-pump oil and neoprene O rings in G x and G 2 and the Bakelite-supported air cushion H beneath the turbine. A thin, light metal disc D insured that the small quantity of vacuum-pump oil leaking through G x could not reach the rotor. The chamber V was evacuated by an oil diffusion pump and forepump through a liquid-nitrogen trap. Variable emf's generated in the liquidmercury contact with the shaft at first gave considerable trouble, but the problem was finally solved by using a very small ...
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