Precision limits of the modern Cavendish device: thermal noise measurement regimes and strategies in the torsion pendulum

The lightly damped torsion pendulum is among the most sensitive of mechanical force detectors. Major limits to its sensitivity arise from horizontal gravitational gradients, seismic disturbance and thermal fluctuations. Unlike the other sources, the more fundamental thermal noise can serve as a common theoretical `standard' against which much of the pendulum's performance can be measured. Nevertheless, its `pure' statistical character from molecular bombardment is not retained through the processes of pendulum action and those of its measurement, as will be shown.