MAINSPRING. The spring which provides the driving power for the going or striking sides of a watch movement.

MAINTAINING POWER. A device for driving a fusee movement by means of a ratchet and click during the action of winding when the power is otherwise taken off.

MALTESE CROSS. A wheel of that shape forming part of a stop-work (q.v.). It is associated with the 'Geneva' stop-work.

MARINE CHRONOMETER. See DETENT ESCAPEMENT.

MAASSEY LEVER ESCAPEMENT. See CRANK LEVER ESCAPEMENT.

MEAN TIME. The time recorded by a watch; that is the average of all the solar days in the year is the ordinary day of 24 hours.

MIDDLE TEMPERATURE ERROR. The elasticity of a balance spring does not vary with temperature changes in the same proportion as the compensating effects of the bimetallic balance (q.v.). A watch with such a balance will only be accurate in its rate for two given temperatures. The inaccuracy between the two extreme temperatures (the M.T.E.) may be corrected by auxiliary compensation.

MINUTE REPEATER. A watch which not only repeats the hours and quarters, but also the minutes which have elapsed since the last quarter. A very few were made in the last quarter of the 18th century and they are not common until the end of the 19th.

MOCK PENDULUM. See FALSE PENDULUM.

MOON HAND. See BREGUET HAND.

MOTION WORK. The gearing under the dial which causes the hour hand to travel twelve times slower than the minute hand. It consists of the cannon pinion, minute wheel and pinion and hour wheel.

MOVEMENT. The main assembly of a watch comprising the power, transmission, escapement, regulating, winding and handsetting mechanisms. In short, the 'works' without the case, hands or dial. Also see ÉBAUCHE.

MUSICAL WATCH. A watch with separate mechanism, set in motion at each hour or at will, which produces a tune on the steel comb and pinned barrel principle. Introduced in Switzerland at the end of the 18th century. Prior to the comb, a set of bells had been used.

NIELLO. A process similar to champleve enamel but using a black metallic filling of silver, lead and sulphur.

NOTCH. See LEVER NOTCH.

NUREMBERG EGG. A misnomer for early South German watches, which in fact were not oval in form but spherical or drum-shape. The term arose from a misreading and mistranslation of 'Uhrlein' into 'Eierlein'. 'little watch' - 'little egg'.

OIL SINK. A small cavity turned in the outside surface of the movement plates around the holes for the pivots to retain the oil. Introduced by Henry Sully in about 1715. Jewels, too, have a sink for oil retention.

OIGNON. The popular name given to the large and rather bulbous French watches of the late 17th and early 18th centuries.

OPENFACE. An 'open face' watch is one without a front cover - i.e. neither a hunter nor a half hunter.

ORMSKIRK. A town in Lancashire. In the early 19th century, a number of watches with a type of Debaufre escapement were made there, and these are known as 'Ormskirk' watches. See DEBAUFRE ESCAPEMENT and DEAD-BEAT VERGE.

OUTER CASE. The outside case in a pair-case watch. Such cases were often embellished.

OVERCOIL. The last coil of a Breguet balance spring. See BALANCE SPRING.