Torsion balances, torsion pendulums, and related devices

Abstract

The torsion pendulum is not only a mainstay instrument in the world of precision measurement and gravitational physics, but is important in electrical science, biophysics, petrology, metallurgy, and various other fields of endeavor. Whether used in the ‘‘static’’ (deflection) mode, the ‘‘dynamic’’ (oscillating) mode, or in some more complex configuration, instrumentation of this kind enables one to isolate and measure weak effects that would otherwise be difficult if not impossible to observe against the background gravitational field of the earth. In this review, we present a brief history of fiber-suspended apparatus and assess the fundamental limits of performance of the dumbbell pendulum. We then inventory the different versions of such systems presently used by gravitational physicists and discuss the various interrogation techniques used to monitor the movement of the suspended test mass. Next, we tabulate some of the applications for torsion instruments outside of gravitational physics, and close with a few comments on the direction of research in this area.

Experimental Limits on the Photon Mass and Cosmic Magnetic Vector Potential

“…1) was used to determine the product Am 2 g . A toroid of electrical steel, of mass 8.4 kg, was wound with 1260 turns of wire carrying 37 mA of current and supported by water flotation [13]. Stability and a restoring torque were provided by a tungsten wire of diameter 0.23 mm and length 198 mm, annealed under tension to reduce drift [14].…”

A specialized torsion balance designed to measure the absolute flux density of hyperthermal molecular beams containing reactive species

“…2 By reflecting a laser beam off a mirror attached to the central axis of the torsion balance onto a position sensitive detector, balance rotations of 10 Ϫ9 rad have been measured. 3 With a lever arm length of 5 cm, these numbers imply that a torsion balance can be used to measure forces of less than 10 Ϫ20 N. Torsion balances are able to accurately measure such small forces because of the mechanical advantage the applied force has over the suspension wire, and because the torque applied to the balance by the gravitational field of the earth is zero.…”

“…3 With a lever arm length of 5 cm, these numbers imply that a torsion balance can be used to measure forces of less than 10 Ϫ20 N. Torsion balances are able to accurately measure such small forces because of the mechanical advantage the applied force has over the suspension wire, and because the torque applied to the balance by the gravitational field of the earth is zero. 2 Torsion balances have been used to determine the absolute flux densities of molecular beams composed of nonreactive gases by measuring the forces exerted by the beams on plates mounted on the lever arms of the balances. The torsion balance described by Bonham and Fink 4 allows the molecular beam to impinge upon a rough flat plate mounted on the torsion balance lever arm.…”

Use of radiation pressure for measurement of high-power laser emission

“…However, all of these previous designs were based on a torsion balance, which has several limitations. These include a slow time response due to long oscillation periods (tens to hundreds of seconds [13]), difficulty in scaling to larger beam diameters, operation in a vacuum environment, and inability for fundamental calibration to a force standard. A recent work [14] achieves impressive results at milliwatt power levels using a disc-pendulum approach but lacks portability and power scalability.…”