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A novel torsion balance
The BIPM is engaged in the determination of the Newtonian constant of gravitation G using a novel torsion balance.
The principal novelties of the BIPM torsion balance are:
a four-mass configuration, rather than the usual two-mass dumbell configuration, to give a much reduced sensitivity to external gravitational perturbing fields; the effect of distant masses falls of as the fifth power of the distance rather than the third power of the distance for a two-mass configuration;
a torsion strip, rather than a torsion wire, to give a much improved stability; this is because the restoring torque of a heavily loaded strip is set mostly by its dimensions and the weight of the suspended object and is practically independent of the material properties of the strip;
a gravitational torque of 2 x
10–8 N · m, which is some four orders of magnitude larger than in most previous torsion balances for determining G; this reduces the effects of non-gravitational perturbing torques, thus improving the ratio of gravitational signal to non-gravitational noise and allowing a very precise measurement to be made in a short time;
three possible configurations of operation: (a) electrostatic servo-controlled, (b) free-deflection and (c) change in period of free oscillation;
permanent installation on a coordinate measuring machine to allow quick and accurate dimensional metrology.
For further details see Richman S.J., Quinn T.J., Speake C.C., Davis R.S., Preliminary determination of G using the BIPM torsion strip balance, Meas. Sci. Technol., 1999, 10, 460-466.
Final results obtained with this apparatus have been published (Quinn T.J., Speake C.C., Richman S.J., Davis R.S., Picard A., A new determination of G using two methods, Phys. Rev. Lett., 2001, 87, 11, article 111101). However, there remains a small but statistically significant discrepancy between the value of G that we have obtained and the two most recent values obtained elsewhere. In order either to confirm our result or to find the source of the discrepancy, we have remade our apparatus with the expectation of reducing our known uncertainties.
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Summary
