BALANCE AND The balance of a watch is a wheel finely poised on its axis; the pivot-holes in which it turns being fre quently—in chronometers and clocks, as well as in watches—jeweled, or made of small rubies, diamonds, etc., for the sake of durability. The natural effect of an impulse given to such a wheel would be a complete rotation on its axis. This, however, is con vertible, by the escapement (q.v.), and by the balance-spring, into a vibratory motion. The balance-spring is held to be a crowning invention in the mechanism of the watch; and the honor of its first suggestion has been claimed for no less than three very eminent men—for Dr. _Hooke, an Englishman; for Abbe Hautefeuille, a Frenchman ; and for lluyghens, the Dutch astronomer. The honor, however, undoubtedly belongs to Hooke.
The balance-spring consists of a coil of steel-wire, so delicately manufactured that 4000 of them scarcely weigli more than one ounce, though often costing more than S21000. In its application to the balance of a watch, one of the extremities of the spring is fastened to a point independent of the balance, while the other is attached near its axis. the balance is at rest, the spring is inclined neither way, this position being called the point of rest; but when the impulse is given to the balance by the crown-wheel of the escapement. the balance moves round just so far as the impulse given is able to overcome the elastic resistance of the spring. When that resistance becomes equal to
the impulse given, the balance stops for a moment, and then is driven back by the elastic recoil of the spring, and continues thus to vibrate so long as the impulse is repeated or the watch is in motion.
The recoil of the spring is sufilcient to drive back the balance to a distance nearly double the length of its first motion; this is, therefore, called the long arc of vibration. But when the motion of the balance is free, with a certain length of spring. the bang arc of vibration is made in less time than the short one, to which the iniptilse is given: with a spring of greater length this relation is reversed; whence it was concluded by Le Roy and Berthoud, that equality of time, or isochronigin, in unequal vibrations, could be more easily obtained by lengthening the spring than by tapering it. In England, where time-keepers have been brought to their greatest perfection, it is considered that isoebro bism is most easily attainable by using the cylindrical helical spring, which is applied to all marine chronometers.
An improvement in watches, or rather in chronometers, invented by Mr. Dent of London, consists in coating the balance and balance-spring with gold by the electro metallurgic process, by which means they are secured from rust.