Period of a Torsion Pendulum as Affected by Adding Weights

Abstract

A precision torsion pendulum with electronic resetting mechanism is described. Prior to each swing, it is held electromagnetically against a stop where it stays for several seconds, held by an automatic time delay. Upon each release, the rotating disk thus has exactly the same starting potential. After a free rotation of several degrees, a beam of light reflected from a mirror on the disk sweeps over a photocell starting a crystal-controlled electronic digital counter. The pendulum then continues clockwise to its maximum excursion and reverses. Upon its counterclockwise return motion it stops the counter as the beam of light sweeps over the photocell in the reverse direction. The recorded times are printed out automatically. They are observed with and without concentric weights added to the rotating disk. In adding weights, there are four factors which act to lengthen the period of the pendulum: the increase in the moment of inertia due to the masses of the added weights, the change in dimensions of the suspending wire, the decreased torsional stiffness of this wire, and the energy used in raising and lowering the disk because of the Poynting effect. The latter three depend on the weights added. When account is taken of these four factors, there is excellent agreement between the observed and computed changes in period with the added loads. Proof is thus furnished that the period of a torsion pendulum is weight sensitive.