Torsion Balance, Coulomb's

Originally an apparatus in which electrostatic attraction or repulsion is measured against the torsion of a filament, often of silk-worm cocoon fibre. It consists in one form of a cylindrical gla ve el in which a light shellac needle is suspended horizontally by a fibre. This needle carries at one end a gilded disc or sphere and is suspended by a fine wire, or filament. A proof plane, q. v., is excited by touching it to the body under trial; it is then inserted in the case. The disc on the needle is first attracted and then repelled. The position finally taken by the needle is noted. The force of torsion thus produced is determined by twisting the filament by the torsion head on the top of the apparatus so as to move the needle a certain distance towards the proof plane. The more the torsion-head has to be turned to carry the needle through a specified arc the greater is the torsion effected or the greater is the repulsion exerted, The torsional force of a wire is proportional to the angle of torsion; this gives the basis for the measurement. With magnetic needle it is used to measure magnetic repulsion and attraction. The best material for the filament is quartz, but the instrument is not very much used.