Physics at Virginia

Finding G

Dr. Beams also applied his knowledge of rotating systems to the measurement of G, the universal constant of gravitation. G was first measured by Henry Cavendish in 1798. Cavendish used a torsion balance to measure the force with which a small lead ball was attracted to a larger ball. In Beams' experiment a cavendish balance is mounted on a table that can rotate about the vertical axis. As the two large masses begin to twist a torsion pendulum, a servomechanism begins to rotate the table at exactly the rate eneded to keep the torsion pendulum from ever getting any nearer the large masses. Throughout each experimental run the rotary table is continuously accelerated; the values of the required acceleration can be used to compute G. The fact that the entire system is rotating tends to cancel out the effects of nearby massive objects on the experiments. Dr. Beams' 1969 experiment was the first substantial change in the method of measuring G since 1942.