T

Tachometer - instrument for measuring speed over a measured distance.  Also called a Tachymeter. Time an event that takes less than 1 minute. The scale on the Tachometer will tell you how many of those events will occur in 1 hour. 
e.g.  A racing car covers 1 mile in 30 seconds. The sweep seconds hand, when stopped at the end of the mile, would point to 120 on the tachometer. The average speed of the car is 120 m.p.h.

Tachymeter - see Tachometer.

Tact Watch - known as the "watch for the blind". The watch has an exposed pointer so that the wearer could determine the time by touch. It is also known as the tactful watch as you could tactfully tell the time in polite society without taking the watch out of your pocket.  see Braille Watch

Tank Watch - a rectangular watch designed by Louis Cartier.  The bars along the side were inspired by the tracks of tanks.

Tantalum - a grey, heavy, and very hard metal. Tantalum is used to make a variety of alloys a high melting point and high strength. Tantalum is completely immune to body liquids and is a non-irritating material. The element is named after Tantalus, in Greek mythology the father of Niobe. One use of Tantalum is in making medical skull plates.

30-minute register -  

the subdial on a chronograph that can 
time periods of up to 30 minutes
 

Time Zone - the world is divided into 24 time zones spaced at intervals of 15 degrees in longitude. The zones start at 0 with Greenwich. Within each time zone, the hour and minute of the day is defined to be the same. Time zones are usually specified by the number of hours they differ from Greenwich Mean Time. For example, Eastern Standard Time (EST) is GMT - 5 hours.

Titanium - a metal, gray in color, that is used for some watch cases and bracelets.  It is much stronger and lighter than stainless steel and is also hypo-allergenic.

Tongue - (tang) Movable metal piece in a buckle which penetrates the holes in a leather strap.

Tonneau watch -

 a watch shaped like a barrel with 2 convex sides.

Tourbillon - (TOOR' bee on) invented by Abraham Louis Breguet in 1795 and patented in 1801 to eliminate errors of rate due to the earth's gravity. It consists of a mobile carriage carrying all the parts of the escapement with the balance in the centre.  Designed to eliminate errors of rate in the vertical positions.  The case makes one revolution per minute compensating for errors in the vertical positions. The tourbillon is considered to be one of the most ingenious horological devices.

Train - The series of wheels in a watch or clock made up of the Center Wheel, Third Wheel, Fourth Wheel and the Escape Wheel.

Tribology - the study of the interaction of sliding surfaces. It includes three subjects: friction, wear, and lubrication. Friction is generally characterized as a branch of physics or mechanical engineering, wear is part of the material science of metallurgy, while lubrication is a branch of chemistry. Tribology is thus a complex interdisciplinary subject.

Tricompax - A chronograph with 3 subdials, traditionally at 3, 6 and 9 o'clock.
                       1. small seconds hand  2. minute counter  3. hour counter.
                       Another configuration has the subdials at 6, 9 and 12 o'clock with the date or day/date at 3 o'clock.

Triple Calendar - a dial that displays the day, date and month.

Tritium - a slightly radio-active substance that collects light and is used to allow the hands or hour markers to glow in the dark. The radiation is so low that there is no health risk.  Watches bearing tritium must be marked as such, with the letter T on the dial near 6 o’clock.

Trimmer - found in expensive quartz movements.  It is a variable capacitor that changes the frequency of the oscillator in order to advance or retard the rate of the watch.

Tropic of Cancer - This line is located 23.5° north of the Equator.
On June 21st, the sun is directly overhead the tropic of Cancer at noon. This is the beginning of summer in the Northern Hemisphere and the beginning of winter in the Southern Hemisphere.

tropic of Capricorn - This line is located 23.5° south of the Equator.
On December 21st, the sun is directly overhead the tropic of Capricorn at noon. This is the beginning of winter in the Northern Hemisphere and the beginning of summer in the Southern Hemisphere.

12-hour Register -  

the subdial on a chronograph that can time 
periods of up to 12 hours.

Two-tone watch - a watch that is made up of two metals. Usually stainless steel and gold.