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Torsion Balance

fibre, force, head, electrical, balls and quartz

TORSION BALANCE, an instrument in which small forces are measured by noting the torsion that they can produce in a fine wire or a delicate fibre of some other material. The invention of the instrument is usually ascribed to Coulomb (1736-1806), who employed it in his extensive researches on electricity. Caven dish also made use of it for the purpose of determining the mass of the earth; his experi ment consisting in determining the attractive power of a pair of leaden spheres, and compar ing this with the attractive power of the earth itself. 'In its conventional form, the torsion balance consists of a light horizontal arm, sus pended at the centre by the fibre whose torsion is to measure the force that is applied to the arm. Quartz is now extensively used for the suspending fibre, its employment having been suggested by C. V. Boys, who showed how to prepare fibres of this material, which are very strong and elastic. Boys dipped an arrow into melted quartz and then shot the arrow from a bow; the quartz being thereby drawn out into a fibre of exceeding fineness. The upper end of the torsion fibre is attached to a graduated head, by whose rotation the fibre can be twisted through a known angle. In applying the torsion balance to the measurement of electrical repul sions, the horizontal arm, f, is provided at one end with a light ball, g, which can be charged to a definite electrical potential, and the torsion head is turned so that this ball is brought to a known distance from a similar fixed ball, g', which can also be charged. The reading of the graduated head being observed when the fibre is fre from torsion and the balls, g g, are at a known distance from each other, the balls are charged. They at once separate, owing to the repulsive action exerted between two electrical charges of the same sign. The graduated head is then turned so as to produce a torsion on the suspending fibre, tending to restore the balls to their original position. The twisting of the

head is continued until the relation of the balls is the same as at first; and when this state is established, it is evident that the torsion of the fibre is exactly balanced by the repulsion of the charges. In order to deduce the electrical re pulsion in definite measure, it is only necessary to determine, by a separate experiment, what force is required to twist the suspending fibre through one entire turn; and a simple propor tion then gives the repulsive force desired. The application of the torsion balance to experi mental work of other kinds will be readily un derstood from the foregoing description of its application to the measurement of electrical re pulsions; for the principles involved are the same in all cases, the force that is to be meas ured being determined by noting the torsion required to neutralize it, in a fibre whose tor sional constant has been determined by direct comparison with a known force. The fact that the torsional moment of a homogeneous twisted fibre is proportional to the angle through which the fibre is twisted was established experimen tally by Coulomb. In actual service the torsion balance is surmounted by a case of metal or glass, the air in which is kept dry by a dish con taining calcium chloride, or phosphorus pent oxide or pumice stone wetted with concentrated sulphuric acid or some other powerful and non-volatile drying agent.

The name "torsion balances has also been applied to a form of commercial balance in which the pans that contain the weights and the objects to be weighed are supported, not upon knife edges, but upon the middle points of nar row, thin, horizontal ribbons of stretched steel. in such a manner that when the' balance de scends at either end, the steel ribbons are exposed to a torsional moment which tends to restore the balance to the normal position of equilibrium.