torsion balance

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An instrument with which small forces, as of electricity or magnetism, are measured by means of the torsion they produce in a wire or slender rod.

GNU Webster's 1913

1. n. (Physics.) an instrument for estimating very minute forces, as electric or magnetic attractions and repulsions, by the torsion of a very slender wire or fiber having at its lower extremity a horizontal bar or needle, upon which the forces act.

WordNet 3.0

1. n. measuring instrument designed to measure small forces by the torsion they exert on a thin wire