portable timepiece that has a movement driven in any of several ways and is designed to be worn (as on the wrist) or carried in the pocket. There can be little doubt that the first watches appeared shortly after 1500, when Peter Henlein, a locksmith in N�rnberg, Ger., introduced the mainspring as a replacement for weights in driving clocks. The escapement used in the early watches was the same as that used in the early clocks, the verge escapement (see escapement). Early watches were made in Germany and at Blois in France. These early timepieces measured some 4 to 5 inches (100 to 125 mm) in diameter and about 3 inches (75 mm) in depth. They were carried about in the hand.
A mainspring consists of a flat spring steel band stressed in bending or coiling; when the watch, or other spring-driven mechanism, is wound, the curvature of the spring is increased, and energy is thus stored. In a watch, this energy is transmitted to the oscillating section of the watch (called the balance) by the wheel train and escapement, the motion of the balance itself controlling the release of the escapement and consequently the timing of the maintaining impulse. A friction drive to the hands is provided from a wheel that rotates at a convenient rate, generally one time per hour. The friction drive permits the hands to be set.
In the first spring-driven timekeepers the mainspring was hooked to an arbor (small shaft) at its centre, or "eye," while its outer end was attached to the frame. A ratchet and click allowed the arbor to be rotated during winding without disturbing the first wheel, or "great wheel," of the train.
One of the main defects of the early watches was the variation in the torque exerted by the mainspring; that is, the force of the mainspring was greater when fully wound than when it was almost run down. Since the timekeeping of a watch fitted with a verge escapement is greatly influenced by the force driving it, this problem was quite serious. Solution of the problem was advanced between 1515 and 1540 by the invention, by Jacob the Czech of Prague, of the fusee, a cone-shaped grooved pulley used together with a barrel containing the mainspring. With this arrangement, the mainspring is made to rotate a barrel in which it is housed; a length of catgut, later replaced by a chain, is wound on it, the other end being coiled around the fusee. When the mainspring is fully wound, the gut or chain pulls on the smallest radius of the cone-shaped fusee; as the mainspring runs down, the leverage is progressively increased as the gut or chain pulls on a larger radius. With correct proportioning of mainspring and fusee radii, an almost constant torque is maintained as the mainspring unwinds. A later invention, the going barrel, a device to keep the watch going during winding, is fitted to all modern watches and has superseded the fusee. By carefully proportioning the barrel arbor (the shaft of the barrel) and barrel diameters to the thickness of the mainspring, torque variations have been reduced to a minimum.
Up to about 1580, the mechanisms of watches were made wholly of iron; about this time, brass was introduced. After about 1625 brass was used for some parts of the watch, and steel for the more delicate pieces.
In the earliest timekeepers, a weighted crossbar (foliot) or a wheel with a heavy rim known as the balance was used to control the rate of going of the mechanism. It was subjected to no systematic constraint, and it was not possible to define its period of oscillation mathematically. Consequently, its period of oscillation, and, hence, the rate of the timekeeper, were dependent on the driving force; this explains the great importance of the fusee.
Controlling the oscillations of a balance with a spring was an important step in the history of timekeeping. Robert Hooke designed a watch with a balance spring in the late 1650s; there appears to be no evidence, however, that the spring was in the form of a spiral, a crucial element that would become widely employed. Christiaan Huygens was probably the first to design (1674-75) a watch with a spiral balance spring, or hairspring. The balance spring is a delicate ribbon of steel or other suitable spring material, generally wound into a spiral form. The inner end is pinned into a collet (a small collar), which fits friction-tight on the balance staff, while the outer end is held in a stud fixed to the movement. This spring acts on the balance as gravity does on the pendulum. If the balance is displaced to one side, the spring is wound and energy stored in it; this energy is then restored to the balance, causing it to swing nearly the same distance to the other side, if the balance is released.
If there were no frictional losses (e.g., air friction, internal friction in the spring material, and friction at the pivots), the balance would swing precisely the same distance to the other side and continue to oscillate indefinitely; because of these losses, however, the oscillations in practice die away. It is the energy stored in the mainspring and fed to the balance through the wheel train and escapement that maintains the oscillations.
The performance of the modern watch depends on the uniformity of the period of oscillation of the balance--i.e., the regularity of its movement. The balance takes the form of a wheel with a heavy rim, while the spring coupled to it provides the restoring torque. The balance possesses inertia, dependent on its mass and configuration. The spring should ideally provide a restoring force directly proportional to the displacement from its unstressed or zero position.
The balance is mounted on a staff or spindle with pivots, and, in watches of good quality, these run in jewels. Two jewels are used at each end of the balance staff, one pierced to provide a bearing, the other a flat end stone providing axial location by bearing against the domed end of the pivot. Frictional effects at the pivots influence the performance of the watch in various positions--for example, lying and hanging.
The balance and spring can be brought to time, or "regulated," by varying either the restoring couple provided by the spring or the inertia of the balance. In the first case (by far the more common), this is generally effected by providing a pair of curb pins mounted on a movable regulator index that lengthen or shorten the hairspring as needed.
In the second instance, screws are provided at one of two pairs of opposite points on the rim of the balance wheel: these screws are friction-tight in their holes and thus can be moved in or out so as to adjust the inertia of the balance. In "free-sprung" watches no regulator index is provided, and the only adjusters are the screws on the balance rim.
Many modern mechanical watches use a lever escapement, invented in 1765 by Thomas Mudge, that leaves the balance free to turn, coupling to it only while receiving the impulse from the mainspring via the wheel train and while unlocking. It was developed into its modern form with the club-toothed escape wheel at the beginning of the 19th century but was not universally adopted until the early 20th century.
In good-quality watches, the club-toothed wheel is made of hardened steel, with the acting surfaces ground and polished; its geometry reduces loss of motion between wheel and pallets. The lever escapement is also characterized by double-roller safety action in which the intersection between the guard pin and roller, which takes place underneath the roller, is much deeper than in early single rollers; thus, any friction caused by jolts encountered in wear causes less constraint on the balance and endangers less the timekeeping properties of the watch. By far the most important watch escapement today--the lever escapement--is used in its jeweled form in watches of moderate to excellent quality; and it is used with steel pallet pins and a simplified fork-and-roller action in cheaper watches (known as pin-pallet watches).
In the wheel train of a modern watch, it is necessary to achieve a step-up ratio of approximately 1 to 4,000 between barrel and escape wheel. This involves four pairs of gears, the ratio per pair commonly being between 6 to 1 and 10 to 1. Because of space considerations, the pinions must have a low number of leaves (projections), commonly 6 to 12. This entails a number of special gearing problems, aggravated by the fineness of the pitch. Any error in centre distance, form, or concentricity is therefore proportionately more important than in larger gear trains.
The first patent covering the application of jewels in watches was taken out in 1704; diamonds and sapphires were used. Synthetic jewels, made from fused powdered alumina (aluminum oxide), are now commonly used. Watch jewels are given a very high polish; a uniform outside diameter for the jewel bearings is highly important, because they are pressed into accurately sized holes smaller than the jewels themselves and held there by friction. The hole diameter of a typical balance jewel is about 0.1 millimetre.
The first patent on the self-winding pocket watch was taken out in London in 1780. An English invention patented in 1924, the self-winding wristwatch contains a swinging weight pivoted at the centre of the movement, coupled to the barrel arbor through reduction wheels and gears. A more modern self-winding watch is fitted with a weight or rotor swinging 360 degrees and winding in both directions.
Electric-powered watches use one of three drive systems: (1) the galvanometer drive, consisting of the conventional balance-hairspring oscillator, kept in motion by the magnetic interaction of a coil and a permanent magnet; (2) the induction drive, in which an electromagnet attracts a balance containing soft magnetic material; or (3) the resonance drive, in which a tiny tuning fork (about 1 inch [25 mm] in length), driven electrically, provides the motive power. Both types (1) and (2) use a mechanical contact, actuated by the balance motion, to provide properly timed electric-drive pulses. Each oscillation of the balance operates a time-indicating gear train by advancing a toothed wheel one tooth. First produced in 1953, type (3), properly called an electronic watch, is inherently more accurate since it operates at a frequency higher than that customarily used with balance-type watches, and the tuning fork is a fairly stable source of frequency. The higher frequency requires the replacement of a mechanical contact by a transistor. The minute and rapid motion of the tuning fork moves forward an extremely fine-toothed ratchet wheel. There is very little friction in the electronic watch; only tiny amounts of oil are needed. When the battery is too weak to operate the tuning fork, the watch simply stops, without deterioration. Miniature high-energy-density batteries are used as power sources in all three types.
The progressive miniaturization of electronic components in the late 20th century made possible the development of all-electronic watches, in which the necessary transistors, resistors, capacitors, and other elements were all on one or several miniature integrated circuits, or chips. The complex circuitry of such watches enabled them to perform a variety of timekeeping functions and also made possible digital readouts of the time in place of the traditional second, minute, and hour hands.
|
|