Torsion, << TAWR shuhn, >> balance is a device for measuring small forces of push or pull. A torsion balance sets an unknown force against the resistance to axial twist in a wire or fiber of small diameter, measuring the twist.
In practice, a torsion balance consists of a fine strand of quartz or sometimes a fine wire of steel or gold. This wire is mounted on a holding mechanism that can be rotated in a horizontal plane, to bring the torsion balance to its zero setting. Suspended from the lower end of the fiber is a horizontal pendulum arm. Each end of the arm carries a ball of a heavy substance that resists corrosion, generally gold, lead, or stainless steel. When the force to be measured is permitted to act on these balls, they swing about, twisting the fiber. A tiny mirror mounted at the junction of fiber and pendulum reflects a beam of light. By noting how much this reflection moves from zero as the force acts, one measures the amount of force in dynes or other convenient units. It is necessary to calibrate the instrument first by twisting it with known forces. These tests must be repeated often, because molecular changes in the fiber may occur.
