Glossary/Arc
Arc
Arc is the full angle through which a pendulum or balance swings. For a precision pendulum this is usually less than 1 1/2° (see Circular Error). Short pendulums may have arcs as wide as 30°. A balance normally swings through 1 1/2 to Q turns, but this falls off to about 1 1/4 turns after the power of the mainspring output has fallen when the spring has run for 24 hours (see Positional Error). Good balance action is described as having 'long arcs' and poor action 'short arcs'.