torsion balance

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun measuring instrument designed to measure small forces by the torsion they exert on a thin wire