TABLE ROLLER. The roller of a lever escapement which carries the impulse pin. The term 'table' was originally applied to distinguish this kind of roller from the part carrying the impulse pin in Massey's escapement, or the so-called 'crank lever'. The impulse pin on the table roller is a jewel and pro-trudes downwards from the table in contradistinction to the crank roller where the impulse piece protrudes from the circumference. The table roller came in shortly after 1825 - 30.
TACT. A 'montre a tact' has a stout external hand fitted over the back cover of the watch, and not connected directly with the movement. If the hand is moved by one's finger in a clockwise direction it moves freely until it is, in fact, indicating the correct time, when it comes up against a stop. The time is then 'read off' by feeling the hand's position relative to pins on the exterior of the case at each hour position. It is thus possible to ascertain the time in the dark. The montre a tact is an inexpensive form of 'repeater'. There is usually a normal dial with two hands visible if the front cover is opened. Breguet made a number of such watches. It is a mistake to think of them as being intended primarily for use by blind persons.
TAILLE DOUCE. Fine line engraving.
TAMBOUR CASE. An early form of watch case, pill box in shape, with hinged cover, cast brass, chiselled and engraved.
TANGENT SCREW. An endless screw, or worm gear. Tangent screw and wheel were used rather before the mid-17th century, replacing the ratchet wheel method, as a means of setting up the mainspring and as a means of regulation. A small dial mounted on the top plate served as an index. After 1675 the worm and wheel were mounted between the plates.
TEMPERATURE COMPENSATION. Any device to counteract the effects of temperature changes on the rate of a watch; in particular, the effect on the steel balance spring which loses some of its elasticity for a rise in temperature with the contrary effect for a fall. See also COMPENSATION BALANCE and COMPENSATION CURB. 18th-century watchmakers usually referred to the 'thermometer'.
TERMINAL CURVES. The curves at the two ends of a cylindrical or helical balance spring, or the outer curve of an overcoil balance spring. In 1858 Edouard Phillips began his investigations into the geometry of the balance spring and into the theoretically correct terminal curves which render it isochronal. Prior to that, watchmakers had used empirical methods, though John Arnold's patent of 1782 referred to the ends of the spring as being 'incurvated'. Briefly, the condition necessary for isochronism is that the centre of gravity of the spring should lie on the axis of the balance. The method adopted to arrive at this condition is to fix the inner and outer ends of the spring in a certain relationship to one another.
THIRD WHEEL. The wheel between the centre and fourth wheel.
THREE-QUARTER PLATE. See FULL PLATE.
TIMING IN POSITIONS. The art of adjusting the balance and its spring so that the watch keeps time in different positions. Also see ADJUSTED.
TIMING SCREWS. Properly, only those screws at the ends of the arms of a cut, compensated balance which are used to bring the watch to time; hence 'mean time screws'. In a chronometer balance there are two timing screws or nuts: one at each end of the arm. Timing screws should not be confused with those balance screws used for adjusting the compensation.
TINTED GOLD. Also called 'coloured gold'. A form of decoration applied to watch cases and dials. The gold is coloured by alloys to produce a reddish, greenish, silvery or yellowish hue. Introduced in the last half of the 18th century it was known as 'or a quatre couleurs', though four colours were not necessarily used. Coloured gold was also used effectively with gems in case decoration.
TIPSY-KEY. See BREGUET KEY.
TOMPION REGULATOR. SEE REGULATION.
TOP PLATE. That furthest from the dial. Also known as the 'potence plate'. The movement plate seen from the back, the other plate being the 'bottom' or 'dial plate'. 'Back plate' is the equivalent plate in a clock movement and is so called.
TOUCH PINS. Pins were usually inserted at the hour positions in 16th century watch dials to enable the time to be read with the aid of a finger during darkness.
TOURBILLON. A. L. Breguet's invention (patented in 1801) for neutralising the positional (vertical) errors inherent in a watch. The escapement is mounted in a revolving carriage or cage with the result that the positional errors are repeated (with a cancelling-out effect) in every revolution of the carriage. The speed of revolution of the carriage depended on the lay-out. In some instances the escape wheel pinion turns about the fixed fourth wheel, the carriage revolving once in a minute, and in others the revolution is made in four or six minutes. A 'tourbillon watch' is not a watch with a special kind of escapement since it may have either a lever or a detent escapement.