Hair Spring
The spring that determines the time of vibrations of the balance. The term hair spring is distinctively American, as all other nations use the more fitting appellation of Balance Spring. The hair spring was invented by Dr. Robert Hooke in 1658, and first applied to a double balance watch for Charles II, on which was inscribed "Robt. Hooke, Inven: 1658. T. Tompion, fecit, 1675." The spring was nearly straight. In 1660 Dr. Hooke altered the form by making it spiral. The different forms of hair springs are illustrated in Fig. 157. The most common form is the volute or spiral spring, shown at A. B is a helical spring used in chronometers. C is a Breguet spring, which is a flat spiral with its outer end bent up above the plane of the body of the spring, and carried in a long curve towards the center, near which it is fixed. The advantage of the Breguet spring is that it distends when in action, on each side of the center, thus relieving the balance pivots of the side pressure which the ordinary flat spring tends to give, and it also offers opportunities of obtaining isochronism by varying the character of the curve. Glasgow says that a hair spring, of whatever form, to be isochronous must satisfy the following conditions: its center of gravity must always be on the axis of the balance, and it must expand and contract in the vibrations concentrically with that axis. Immish contends that mere length of spring has nothing to do with isochronism. Mr. Glasgow contends that the whole question of isochronism resolves itself into the adoption of a spring of the correct length, and recommends for a lever watch fourteen turns if a flat, and twenty turns if a Breguet spring is used, if a cylinder watch use from eight to twelve turns. He argues that if a spring is too short, the short vibrations will be fast and the long vibrations slow, and that all bending and manipulation of the spring with a view to obtaining isochronism are really only attempts to alter the effective length of the spring. Mr. Britten contends that the position of the points of attachment of the inner and outer turns of a hair spring in relation to each other has an effect on the long and short vibrations quite apart from its length. For instance, a very different performance may be obtained with two springs of precisely the same length and character in other respects, but pinned in so that one has exactly complete turns, and the other a little under or a little over complete turns. He argues that a short spring as a rule requires to be pinned in short or complete turns, and a long one beyond the complete turns. In duplex and other watches with frictional escapements, small arcs of vibration and short springs, it will be found that the spring requires to be pinned in nearly half a turn short of complete turns.