BACK PLATE. See TOP PLATE.

BALANCE. A plain wheel with two or three spokes known as 'arms'. Coupled to its spring, the balance of a watch is the controlling device; its oscillation, its to and from swinging properties, regularise the movement of the train powered by the mainspring, and hence the timekeeping ability of the watch. A plain balance, or a monometallic balance, is an uncut ring, with or without timing screws. It may be made of brass, steel, gold, nickel or palladium. A modern alloyed metal such as 'Invar' or glucydur produces a balance which for all practical purposes is unaffected by temperature changes.
A 'compensated', cut bimetallic balance has its rim made of two metals of different coefficient of expansion (brass and steel) fused together. The rim is cut near each arm of the two-arm balance, the other end, the free end, moves inward or outward on a rise or fall (respectively) in temperature, thus altering the moment of inertia of the balance by shifting the mass of the rim closer to or away from the centre. This counteracts the effect of temperature charges upon the elasticity of the steel balance spring. Brass has a higher coefficient of expansion than steel, and is on the outer side of the rim. Screws on the balance rim enable the compensating property to be adjusted. An 'unsprung balance' is a balance without a spring - i.e. before the introduction of the balance spring in 1675. See also FOLIOT and ' S ' BALANCE.

BALANCE COCK. See COCK.

BALANCE SCREWS. See TIMING SCREWS.

BALANCE SPRING. Both Robert Hooke and Christiaan Huygens have been credited with the invention of the balance spring as the controlling agent for watches. The Abbe de Hautefeuille also claimed priority, though it seems fairly certain that his conception was for a straight as opposed to a spiral spring. It also appears that Hooke's original idea of 1658 was a straight spring. It is certainly established beyond doubt that Huygens devised a practical means of applying a spiral spring and employed Thuret of Paris to make a watch with such a spring in 1675. Hooke had in 1664 propounded the first law relating to springs: ut tensio sic vis ('as the tension is, so is the force'). Put more simply: 'The force which a spring exerts depends upon the amount it has been wound up.' Hooke's Law is only partially true as applied to a watch balance spring.
A balance spring is a weak spiral spring attached at its inner end to the balance staff and at its outer end to the balance cock or movement plate. The sprung balance regulates the timekeeping, the period of each swing depending upon the ratio of the moment of inertia of the balance to the stiffness or elasticity of the spring. The elasticity of any given spring depends basically upon the material from which it is made and upon its effective length. See ISOCHRONOUS, COMPENSATED BALANCE and COMPENSATION CURB.
The earliest balance springs were made of copper or iron, later of steel. Gold springs were occasionally used in the later 18th and 19th centuries. C. A. Paillard invented a palladium spring - which is non-magnetic - in 1877, and of recent years alloys of nickel steel, chromium, manganese and other elements have rendered balance springs impervious to temperature changes, magnetic fields and damp. Dr C. E. Guillaume did the original research on nickel steel alloys in 1896. 'Elinvar' ('elasticite invariable') was the name given to his springs. The earliest springs (with the verge escapement) had three or four coils; with the later escapements about fourteen are general.
A helical spring is one formed into a helix and is normally found with the chronometer or detent escapement. In about 1782 John Arnold discovered by empirical methods that isochronism could be achieved by a helical spring with the two terminal coils or ends incurved. A. L. Breguet introduced a spiral spring with an overcoil as an aid to isochronism. The overcoil spring has the outer coil raised and turned in towards the centre, which ensures the concentric development of the spring as the balance oscillates. In 1861, Edouard Phillips gave the theoretical conditions for end curves for both helical and spiral springs to render them isochronous. Jules Grossman and L. Lossier took these investigations further.
A steel spring will lose its elasticity in heat and become more 'springy' in cold; hence the need for temperature compensation.

BALANCE STAFF. The spindle or arbor upon which the balance is mounted.

BANKING PIN. With the verge escapement, a pin protruding from the outer edge of the balance. The extreme arc of balance swing is limited by stops on the balance cock, against which the pin would 'bank' if the arc were excessive. In later escapements, an equivalent provision is included and in the lever escapement the device of two banking pins is used to limit the angular movement of the lever. Occasionally, instead of two pins, banking takes place against walls forming part of the movement plate or of the pallet cock.

BAR. Bridge as distinct from a cock which has a foot by which it is secured to the movement plate.

BARREL (GOING). A cylindrical box (barrel) with a toothed disc (a wheel) on the outer edge. The disc is the 'great wheel' and the box contains the mainspring. The barrel (box) turns freely on its arbor, the mainspring being hooked to the barrel at its outer end and to the arbor at its inner end. The great wheel meshes wit!i the first pinion of the watch train. In a watch movement with going barrel, the fusee (q.v.) is dispensed with. The barrel in fusee watches is a plain barrel without teeth. In winding a going barrel, the barrel arbor is turned round, drawing the spring away from the rim of the box and coiling it round the arbor. A lick and ratchet prevent the arbor from recoiling while the mainspring is being wound and when it is fully wound. The tensioned spring, in striving to unwind', expends its force in turning the barrel, his same force being utilised to drive the watch rain via the great wheel. The barrel makes as many turns in unwinding, of course, as were given to the arbor in winding.
See also STOP WORK and SET UP.

BARREL (HANGING). A going barrel, fixed to the movement only by its upper portion under a bridge or cock. Also known as 'standing barrel'.

BARREL (RESTING). The great wheel is mounted m an arbor, the power transmitted to the wheel by ratchet and click. The spring is enclosed in a barrel which is screwed to the plate.

BARROW'S REGULATOR. An early form of regula-or of balance spring watches. Two pins (curb pins) held upright in a slide embrace the end section of he balance spring which is straight, not coiled. The slide moves along a worm (endless screw) which has a squared end to take a key. An index engraved on he movement plate indicates the amount the slide nay be moved with the aid of a key, as the effective length of the spring is altered for regulation. A watch still retaining its Barrow regulator is very rare.

BASSE-TAILLE ENAMEL. Translucent enamel laid over a ground engraved to enhance the pictorial effect.

BASSINE. A type of watch case that is rounded on the edge and smooth.