'S' BALANCE. An early form of bimetallic compensation balance used by John Arnold in 1779 - 82. The bimetallic 'strips' were formed into two elongated 'S' laminated pieces whose purpose and effect was to move inwards or outwards (in heat or cold) two weights situated on opposite sides of the balance rim.

SAFETY ROLLER. See ROLLER.

SAFETY DART. See DART.

SAVAGE TWO-PIN. A form of lever escapement named after George Savage in which the roller carries two pins which unlock the escapement via the fork. A third pin, mounted upright at the end of the lever, acts as the impulse pin, passing into a narrow notch cut in the roller, when the escape wheel has been unlocked. This third pin also serves as the guard pin for the safety action during the supplementary arc. Savage introduced his escapement about 1814, certainly before 1818 when he emigrated to Canada. There he founded the firm of Savage & Lyman. Very few watches signed by Savage have survived. He made for Edward Brace-bridge of Red Lion Street, 1805 - 15.

SAVONNETTE. A watch with a front cover to give protection to the glass. See HUNTER.

SECONDS PINION. The extension of the fourth wheel arbor to which the seconds hand is attached.

SECRET SIGNATURE. A device used by A. L. Breguet to combat the forgery of his work. The name and the number of the watch were engraved with a pantograph on the dial, and these can only be seen when examining the dial with the light falling across it. Subsequently, makers other than Breguet used the idea.

SECRET SPRING. The fly and lock springs of a hunter watch case.

SELF-COMPENSATING SPRING. A balance spring made of an alloy impervious to temperature changes.

SELF-WINDING. See PERPETUAL WATCH.

SET - UP. The degree of tension to which a mainspring is set when the watch is fully run down. The use of stop-work enables a going barrel watch to be 'set-up' so that only the middle turns of the mainspring are in use, thus providing more even torque. By altering the set-up a degree of regulation was effected in the fusee, pre-balance spring watch.

SHIP'S CHRONOMETER. See DETENT ESCAPEMENT.

SINGLE ROLLER. See TABLE ROLLER.

SIX-HOUR DIAL. A watch dial of the late 17th-century period in which the chapter ring is marked in roman numerals I to VI and superimposed on these are arabic numerals 7 to 12. The single hand revolves once in six hours. Due to the larger spacing between the numerals (only six instead of twelve) the divisions between them can be legibly calibrated into two-minute divisions, and the time read to two minutes from the one hand.

SKELETON DIAL. A dial from which metal has been cut away leaving only the hour and minute ring, thus exposing the movement.

SKELETONISED MOVEMENT. A watch movement in which all the spare metal has been cut away from the top movement plate, thus exposing the wheel work.

SNAIL. A cam shaped like the profile of a snail; part of the striking mechanism in the rack-striking layout. The snail determines - owing to the steps cut thereon - how far the rack may fall when it is released, and hence how many blows are struck, each step corresponding to an hour.

SOUSCRIPTION (MONTRE A ). The cheapest possible Breguet watch of the highest possible quality, subscribed for in advance and made by him in batches.

SPLIT-SECONDS. A form of chronograph in which there are two centre seconds hands, one over the other. When the chronograph is started the hands travel together, but by operating a push-piece the under one is halted while the other continues. The stationary hand can be made to rejoin its fellow, or the second of the two hands can also be stopped. A third pressure returns the hands to zero.

SPRING BARREL. The barrel containing the mainspring.

SPRING DETENT. In the chronometer escapement a blade spring carrying the locking jewel. A detent mounted on a spring. It is sometimes called a 'footed detent'. Patented by John Arnold in 1782.

STACKFREED. An eccentric cam or snail serving as a mainspring-equaliser. It is mounted on a wheel geared to a pinion on the mainspring arbor, making rather less than one revolution on one winding or in the going time of the watch. Pressing against the edge of the snail is a strong spring which terminates in a roller, the roller thus setting up friction between itself and the edge of the snail or, more correctly, between the stackfreed wheel and the post upon which it revolves, the friction varying with the radius of the snail. The wheel upon which the snail is mounted has a short section on which no teeth are cut. This, in conjunction with the pinion on the mainspring arbor, provides stop-work. When the watch is run down, the roller lies in a notch or depression between the highest and lowest portions of the snail. In this position the roller-spring is not tensioned. Also, the uncut section of the snail-wheel butts against a leaf of the pinion mounted on the mainspring arbor. On winding the watch, the 'stop' position is reached when the pinion comes up against the uncut section of the snail-wheel, approaching it from the contrary direction. Also, on winding, the roller is lifted out of the notch on to the snail at its highest point where it exerts greatest pressure. As the mainspring runs down, less and less pressure is exerted to match the decreasing force of the mainspring until, right at the end of the run, the roller starts to enter the notch, and for a short time actually 'helps' the mainspring.The stop-work arrangement not only prevents over-winding, but - and this is important - can be arranged to allow only a portion of the mainspring to be used; that is, the middle portion, thus avoiding the two extremes. (See SET - UP.)

The stackfreed was almost certainly invented in Nuremberg, probably fairly early in the 16th century. It would appear that Henlein's watches with a 40 hour duration employed neither the stackfreed nor fusee. A few watches with a wedge-shaped cam and without a notch exist. In this type - which is the late form of stackfreed - the mainspring was given 'assistance' rather sooner than on the snail-type. The profile of the cam altered over the years, starting from a simple curve, with the notch, and becoming more 'snail'-like. The form of the cam probably altered as it became possible to make thicker (consequently shorter) springs which would produce a different torque-curve. The final form (circa 1585 - 1620), before the 'wedge', was kidney-shaped. The stackfreed seems to have been confined to southern Germany. Its only advantage over the early, crude fusee was that it occupied less height and so enabled a flatter watch to be made.

STANDING BARREL. Hanging barrel is the alternative name for a going barrel whose arbor is supported at the upper end only.

STOP-WORK. A device to prevent overwinding of a mainspring, and to enable the spring to be set-up. (See SET - UP) A form of stop-work was used on the earliest Nuremberg watches. See STACKFREED.

STRAIGHT LINE LEVER. A lever escapement in which the escape wheel arbor, the pallet staff and the balance staff are planted in a straight line.

STRIP COMPENSATION. See COMPENSATION CURB.

STUD. A small piece of metal pierced to receive the end of the outer coil of a balance spring.

SUGAR TONGS. The name given to the compensation curb used by Thomas Earnshaw, its shape roughly resembling a pair of tongs.

SUN AND MOON DIAL. A popular dial in the late 17th and early 18th century. A sun is depicted on one half and a moon on the other of a disc revolving once in 24 hours. Each is visible in turn through a semi circular hole cut in the dial plate. Above the hole a segment of the dial is marked from VII to XII and from I to VI. The sun indicates 6 a.m. to 6 p.m. and the moon 6 p.m. to 6 a.m. The minutes are indicated in the normal way.

SUPPLEMENTARY ARC. The arc described by an oscillating balance outside the function of escapement, after the impulse and before unlocking.

SURPRISE PIECE. A device fitted to the quarter-snail of a repeater to prevent incorrect striking just prior to the next hour.

SWEEP SECONDS. See CENTRE SECONDS.

SWING WHEEL. The escape wheel used to be so called.

SWISS LEVER ESCAPEMENT. The lever escapement now in universal use. It is distinguished by the form of the escape wheel teeth - i.e. club-toothed.