Figure 6.2: Torsion pendulum used in the experiment of Su, et al. (1994) to test the equality of accelerations for test bodies attracted by the Earth, the Sun, and the matter in our galaxy. The pendulum is small (its overall diameter is about 3 in.) to minimize disturbing effects from local variations in the gravitational force. It hangs from a tungsten fiber, which is so thin that it cannot be seen in this photograph. The circular plate holds four cylindrical test bodies (two of copper and two of beryllium) along with four right-angle mirrors that are part of a sensitive optical system for detecting pendulum twists. The pendulum is suspended in a vacuum and the entire instrument is rotated continuously at about one revolution per hour. A violation of the equality of gravitational and inertial mass would show up as a pendulum twist that varied at this rotation frequency.