What is a Torsion Balance?

  A Simple Composition Dipole

Torsion balance experiments in general measure some quantity by detecting a torque on a hanging pendulum; the torque is produced by some field interacting with a dipole or higher order moment. In some of our balances, we exploit a composition dipole to test for composition-dependent forces. For example, we constructed a dipole from Cu and Pb test bodies for our Rot-Wash experiment.

The pendulum sits in the field of an attractor; either the pendulum or the attractor mass is rotated so that an oscillating torque (easier to identify then a static torque) is produced.

One very nice feature of torsion balance experiments is that they are insensitive to slight differences in the masses or moment arms of the test bodies. Such differences will tilt the pendulum so that any torques that could be caused by the mass or moment arm differences cancel.


What does Eöt-Wash mean?

Our group's name is a pun on the name of a famous gravitational physicist, the Hungarian Count von Eötvös (pronounced something like ootvoosh with oo as in foot). Eötvös pioneered the use of torsion balances for testing the principle of equivalence, i.e. is gravitational mass (the mass entering in Newton's law of gravity F=G mm/r 2) exactly the same as inertial mass (the mass responsible for inertia in Newton's mechanical law F=ma). The suffix '-Wash' indicates that we are doing torsion balance tests of gravity at the University of Washington.

Eöt-Wash Group Contact

For more information, contact:
Eric Adelberger at (206) 543-4294 or eric@npl.washington.edu
Blayne Heckel   at (206) 685-2401 or heckel@phys.washington.edu
Jens Gundlach  at (206) 616-2960 or gundlach@npl.washington.edu