**T [1]**- informal abbreviation for "trillion," meaning the American trillion 10
^{12}. **T [2]**- symbol for the Dvorak T-number, a subjective estimate of the strength of a tropical cyclone based on its appearance in satellite imagery. The T-numbers range from T1.0 to T8.0 in steps of 0.5; ratings above T2.5 indicate a tropical storm and ratings above T4.0 indicate a hurricane or typhoon. The scale was developed by the U.S. meteorologist Vernon Dvorak in 1974. Wikipedia has details.
**T [3]**- the symbol for the tesla (see below).
**table**- an informal unit of quantity equal to 4, the number of players needed for certain card games, especially bridge.
**tablespoon or tablespoonful (tbsp, tblsp, Tsp or T) [1]**- a unit of volume used in food recipes. In the U.S., the tablespoon is equal to 1/2 fluid ounce; this is about 14.8 milliliters. In Canada, the traditional tablespoon is 1/2 Imperial fluid ounce (14.2 milliliters). In Britain, traditional tablespoons varied somewhat in size, and various older references give sizes in the range from 1/2 to 5/8 Imperial fluid ounce (14.2-17.6 milliliters). Under the metric system the tablespoon has become more or less standardized at 15 milliliters in Britain, Canada, and New Zealand, 20 milliliters in Australia. The U.S. tablespoon equals 3 teaspoons or 1/16 cup; the traditional British tablespoonful was often equal to 4 teaspoonfuls or 1/10 teacupful. The metric tablespoon equals 3 teaspoons (4 in Australia).
**tablespoon or tablespoonful (tbsp, tblsp, Tsp, or T) [2]**- a unit of volume used in
**tael or tahil**- a traditional unit of weight used throughout eastern Asia. During the colonial period, the tael was more or less standardized throughout the region at 4/3 ounce avoirdupois (1/16 catty, 1/12 pound, or about 37.8 grams). In Japan, however, the tael was identified with a slightly smaller traditional unit and is considered equal to 1.323 ounces (37.51 grams). The tael is usually considered equal to the Chinese liang.
**tagwerk**- see juchart.
**talangwah (tw)**- the square wah, a common unit of area in Thailand, equal to exactly 4 square meters or 43.0556 square feet. There are 100 talangwah in 1 ngarn, 400 in 1 rai.
**talbot (T)**- a unit of luminous (light) energy. One talbot is the energy carried by a
light flux of one lumen in one second, that
is, the talbot is the same as the lumen second (lm·s). For light of wavelength
555 nanometers (nm), the wavelength to which the eye is most sensitive, the
talbot equals 1.464 millijoule. For other wavelengths
*l*, the talbot equals 1.464·*V*(*l*) millijoules, where*V*(*l*) is the "luminous efficiency," a factor representing the relative sensitivity of the eye at wavelength*l*. Although the talbot is compatible with the SI, it has not been accepted as part of the International System; the symbol T would not be acceptable since it duplicates the symbol for the tesla. The unit, previously called the**lumberg**, is now named for the British physicist W.H. Fox Talbot (1800-1877). **talent**- a historic unit of weight, used in various forms throughout the eastern Mediterranean. The Hebrew sacred talent, mentioned in the Bible, was equal to 60 minas or about 30 kilograms (66 pounds). The Greek talent, also equal to 60 minas, was smaller, 25.8 kilograms or about 57 pounds.
**tan [1]**- a traditional Chinese weight unit, now spelled
**dan**in English transliteration. During the European colonial era the tan was equal to 100 cattys or 1600 taels. This is equivalent to 133.333 pounds, making the tan comparable to the European quintal as a commercial weight unit. In modern China the tan, or rather the dan, is equal to 100 jin, which is exactly 50 kilograms (110.231 pounds). **tan [2]**- a traditional unit of land area in Japan equal to 10 se or about 991.7 square meters (0.099 hectare or 0.245 acre).
**tarea**- a traditional unit of land area in the Dominican Republic and some parts
of Central America. The tarea is generally equal to 900 square varas,
so it is the area of a square 30 varas (about 25.05 meters) on a side. In
the Dominican Republic, the tarea equals about 628 square meters or 751 square
yards; this is 0.0628 hectare or 0.155 acre.
The Spanish word
*tarea*means a task or job, so this unit, like many land area units, originated as the area which could be "worked" in a given time. **tarefa**- a traditional unit of land area in Brazil, varying in size from one locality
to another but generally at least 3000 square meters (0.3 hectare
or 0.741 acre). In the states of Alagoas, Rio
Grande do Norte, and Sergipe the tarefa equals 3025 square meters (0.3025
hectare or 0.748 acre);
in Ceará the unit equals 3630 square meters (0.363 hectare
or 0.897 acre); and in Bahia the tarefa is 4356
square meters (0.4356 hectare or 1.076 acre).
The Portuguese word
*tarefa*, like the Spanish*tarea*, means a task or job. **tatami**- a Japanese unit of area equal to the area of a traditional tatami mat, 0.5
ken by 1 ken or about 90 centimeters by 180 centimeters
(roughly 1.62 square meters or 17.5 square feet). This unit, used especially
for measuring the area of rooms in houses and apartments, is also called the
**jo**. **tce**- a symbol for
**tonne of coal equivalent**, a unit of energy used in the international energy industry. 1 tce represents the energy available from burning one tonne (metric ton) of coal; this is considered equivalent to exactly 0.7 tonnes or approximately 5.2 barrels of oil, or 890 cubic meters of natural gas. 1 tce is equivalent to 29.308 gigajoules (GJ), 27.778 million Btu (MM Btu) or dekatherms, or 8.141 megawatt hours (MWh). **Te, te**- non-standard symbols for the tonne (metric ton), used by some engineers
in the UK. The only proper symbol for the tonne is
**t**. If it is necessary to distinguish the tonne from the British Imperial ton, use**tn**for the British unit. **teacupful**- a unit of liquid volume used in British food recipes. The teacupful is the same volume as an Imperial gill: 5 fluid ounces, 8.670 cubic inches, or about 137.7 milliliters.
**teaspoon or teaspoonful (tsp or t) [1]**- a unit of volume used in food recipes. The U.S. teaspoon is equal to 1/3
tablespoon or 1/48 cup; this is equivalent
to 1/6 fluid ounce, about 0.30 cubic
inches, or approximately 4.9 milliliters. In Canada, the traditional teaspoon
is 1/6 Imperial fluid ounce or about 4.74 milliliters. In Britain, a traditional
**teaspoonful**in the kitchen was equal to 1/8 Imperial fluid ounce or approximately 3.55 milliliters, but the medical teaspoonful was usually 5 milliliters. In metric kitchens in Britain, Canada, Australia, and New Zealand, a teaspoonful is exactly 5 milliliters. **teaspoon or teaspoonful (tsp or t) [2]**- a unit of volume used in
**technical atmosphere (at)**- a metric unit of pressure equal to one kilogram
of force per square centimeter. The technical atmosphere equals about
980.665 millibars (mb), 98.0665 kilopascals
(kPa), approximately 28.96 inches of mercury
(in Hg), or 14.223 pounds of force per square
inch (lbf/in
^{2}). This is about 97% of the average pressure of the earth's atmosphere at sea level. **tebi- (Ti-)**- a binary prefix meaning 2
^{40}= 1 099 511 627 776. This prefix was adopted by the International Electrotechnical Commission in 1998 to replace tera- for binary applications in computer science. In particular a**tebibyte (TiB)**is 2^{40}= 1 099 511 627 776 bytes. The prefix is a contraction of "terabinary." **tenth-meter**- an old name for the angstrom, a unit of
distance equal to 10
^{-10}meter. **tera- (T-)**- a metric prefix meaning 10
^{12}, or one trillion. The prefix was derived from the Greek word for monster,*teras*. (Our words "terrible" and "terrific" have this same root.) The three prefixes mega-, giga-, and tera- thus mean something like "huge," "gigantic," and "monstrous," respectively. But also, and apparently by coincidence, the prefix tera- suggests the Greek*tetra*, meaning 4. This is significant because tera- is the fourth prefix, following kilo-, mega-, and giga-, in the list of SI prefixes for multiples of 1000^{n}. This coincidence was exploited in defining the subsequent prefixes peta-, exa-, zetta-, and yotta-, corresponding to the powers*n*= 5 through 8 in 1000^{n}, so that they suggest the Greek numbers*penta*,*hexa*,*hepta*, and*okta*for 5, 6, 7, and 8, respectively. **terabecquerel (TBq)**- a unit of radioactivity equal to 10
^{12}atomic disintegrations per second or 27.027 curies. **terabyte (TB)**- a unit of information equal to 10
^{12}(one trillion) bytes. The terabyte is often used to mean 2^{40}= 1 099 511 627 776 bytes but this use breaks the rules of the SI. The unit equal to 2^{40}bytes should be called the**tebibyte**(see above). **teraflops (Tflops)**- a unit of computing power equal to one trillion (10
^{12}) floating point operations per second. See flops. **teragram (Tg)**- a metric unit of mass equal to 10
^{12}grams or 1 megatonne (one million metric tons). This unit is frequently used in atmospheric science and other scientific contexts where large masses are considered. **terahertz (THz)**- a unit of frequency equal to 10
^{12}per second or 1 per picosecond. Infrared and visible light waves have frequencies measured in terahertz. **terajoule (TJ)**- a metric unit of energy commonly used in the energy industry, equal to 10
^{12}joules. One terajoule equals 947.817 million Btu, 277.7778 megawatt hours (MW·h), or about 9480 therms (see below). **terameter (Tm)**- a metric unit of distance equal to 10
^{12}meters or 10^{9}kilometers. This is about 6.6846 astronomical units. The distance from Saturn to the Sun is about 1.43 terameters. **terannual**- an adjective meaning 3 times per year or once every 4 months. Not to be confused with triennial (once every 3 years).
**terawatt (TW)**- a metric unit of power equal to one trillion (10
^{12}) watts or about 1.341 billion horsepower. **terawatt hour (TW·h)**- a metric unit of energy equal to one billion kilowatt hours (kW·h), 3.6 petajoules (PJ), or about 3.412 trillion Btu.
**tertian**- a traditional unit of volume for liquids. The tertian, like the tierce (see below) takes its name from the Latin for 1/3. It equals 1/3 tun, which is 2 tierces or 84 U.S. gallons (about 318 liters).
**tesla (T)**- the SI unit of flux density (or field intensity) for magnetic fields (also called the magnetic induction). The intensity of a magnetic field can be measured by placing a current-carrying conductor in the field. The magnetic field exerts a force on the conductor, a force which depends on the amount of the current and on the length of the conductor. One tesla is defined as the field intensity generating one newton of force per ampere of current per meter of conductor. Equivalently, one tesla represents a magnetic flux density of one weber per square meter of area. A field of one tesla is quite strong: the strongest fields available in laboratories are about 20 teslas, and the Earth's magnetic flux density, at its surface, is about 50 microteslas (µT). One tesla equals 10 000 gauss. The tesla, defined in 1958, honors the Serbian-American electrical engineer Nikola Tesla (1856-1943), whose work in electromagnetic induction led to the first practical generators and motors using alternating current.
**tetrad [1]**- a unit of quantity equal to 4.
**tetrad [2]**- a unit of digital information equal to 4 bits or 1/2 byte.
This unit, in various contexts, is also called a
**nibble**, a**quadbit**, or a**hexit**. The unit seems to be more common in German, where it is spelled**tetrade**. **TEU**- a unit of cargo capacity, especially for container ships. These ships carry cargo in standard metal boxes, called containers, which can be transferred easily to trains or trucks. TEU is an abbreviation for "twenty-foot equivalent unit." One TEU represents the cargo capacity of a standard container 20 feet long, 8 feet wide, and (usually) a little over 8 feet high, or half the capacity of a similar container 40 feet long. One TEU equals about 12 register tons (see ton [3] below) or 34 cubic meters.
**TeV**- the symbol for one trillion (10
^{12}) electronvolts. Thanks to Einstein's equation*E = mc*equating mass with energy, the TeV can be regarded either as a unit of energy equal to 160.217 646 2 nanojoules, or as a unit of mass equal to 1.782 662 x 10^{2}^{-21}gram or 1073.544 atomic mass units. **tex**- a metric unit used in the textile industry to measure the density of a single fiber of yarn. One tex equals a density of one gram per kilometer of length, or 1 mg/m. One tex equals 10 drex or 9 denier.
**therblig**- a unit of physical activity used in time-and-motion studies in industrial engineering. A therblig represents one of 18 standardized activities identified by the American industrial psychologists Frank B. Gilbreth (1868-1924) and Lillian Moller Gilbreth (1878-1972): search, find, select, grasp, hold, position, assemble, use, disassemble, inspect, transport loaded, transport unloaded, pre-position for next operation, release load, unavoidable delay, avoidable delay, plan, and rest for overcoming fatigue. The word is Gilbreth spelled backwards (considering "th" as one letter).
**therm (thm)**- a commercial unit of heat energy. The therm is equal to 100 000 Btu.
Because there have been several definitions of the Btu, there are two official
definitions of the therm. In the U.S., the legal definition (made in 1968)
is that the therm equals 105.4804 megajoules.
The European Union's definition, made in 1979 using the more current IT Btu,
is 105.5060 megajoules. Either way the therm is equal to about 25 200 (large)
calories or about 29.3 kilowatt
hours of electrical energy. One therm can also be provided by about 96.7
cubic feet of natural gas. The therm has sometimes been confused with the
thermie (see below). The names of both units come from the Greek word for
heat,
*therme*. **thermal ohm**- an informal name for the SI unit of thermal resistance, kelvins per watt (K/W) or degrees Celsius per watt (°C/W). The thermal resistance of an insulating material, in thermal ohms, is the R-value (in SI units, equal to 0.1442 times the R-value in English units) divided by the thickness of the material, in meters. The name expresses an analogy between heat flow and electric current: the temperature difference (in kelvins or °C) corresponds to electric potential difference (in volts), while the heat flow rate (in watts, equal to joules per second) corresponds to electric current (in amperes). By Ohm's Law, the ratio of potential to current is the electric resistance (measured in ohms). Thus the corresponding ratio of temperature difference to heat flow rate is measured in thermal ohms.
**thermie (th)**- a metric unit of heat energy, part of the meter-tonne-second system sometimes used by European engineers. The thermie is equal to the amount of energy required to raise the temperature of 1 tonne of water by 1°C. The thermie is equivalent to 1000 (large) calories, 4.1868 megajoules or 3968.3 Btu.
**thermochemical calorie (cal**_{th})- a form of the calorie formerly used in chemistry. The thermochemical calorie was defined in 1935 to equal exactly 4.184 joules. This is slightly smaller than the International Steam Table calorie (4.1868 joules) defined in 1956.
**thimbleful**- an informal unit of volume, often used as a prototypical small amount in statements such as "a thimbleful of matter from a neutron star would weigh 100 million tons." A thimble holds just about one cubic centimeter (or one milliliter).
**third [1]**- a unit used in music to describe the ratio in frequency between notes. Two
notes differ by a
**minor third**if the higher note has frequency exactly 6/5 times the frequency of the lower one, or by a**major third**if the higher note has frequency exactly 5/4 times the frequency of the lower one. On the standard 12-tone scale, the minor third is approximated by 3 half steps, corresponding to a frequency ratio of 2^{1/4}= 1.1892; the major third is approximated by 4 half steps, corresponding to a frequency ratio of 2^{1/3}= 1.2599. **third ("') [2]**- a unit of time equal to 1/60 second. The symbol for this rarely-used unit is a triple prime, suggesting a natural progression from the minute (') through the second (") to the third ("').
**third-octave**- a unit of frequency interval describing a band of frequencies such that
the highest frequency is 2
^{1/3}= 1.260 times the lowest. This unit is commonly used in noise control and abatement. **thou [1]**- an alternate name for what Americans call a mil: a unit of distance equal to 0.001 inch (25.4 micrometers). This name originated in Britain, but it is now common in the U.S. also.
**thou [2]**- an informal contraction of "thousand," sometimes used as a unit of quantity.
**thrave**- a traditional measure of grain in Scotland and northern England. A thrave is usually 24 sheaves, each about 30 inches (76 centimeters) in circumference.
**thread**- a traditional unit of length for cotton yarn, equal to 54 inches (1.5 yards, or 137.16 centimeters). There are 80 threads in a skein or lea.
**thumb**- another name for an inch.
**tical**- a traditional unit of weight in southeast Asia, originating as the weight of a coin of the same name. In Myanmar (Burma), where the unit is still in use for measuring the weights of precious metals and drugs, the tical is equivalent to 16.4 grams.
**tick [1]**- an informal unit of time equal to the length of one cycle of a clock. A tick of a computer's system clock, also called a jiffy, is usually 0.01 second. A tick in athletics is the smallest increment of time measured in a timed competition, usually 0.1 second or 0.01 second.
**tick [2]**- a unit used in finance and investing to express the smallest measured change in a price or index. For example, at the Chicago Board of Trade in the U.S. a tick in the price of many agricultural commodities is equal to 1/4 cent ($0.0025) per bushel [3].
**TID**- abbreviation for the Latin
*ter in die*, three times a day, a unit of frequency traditionally used in medical prescriptions. **tidal day**- a unit of time equal to the average period between two successive passages
of the moon through the meridian (the imaginary line across the sky from
due north to due south). High and low tides repeat with this period (on the
average), so the unit is fundamental to predictions of these tides. The tidal
day, also called the
**lunar day**, is equal to approximately 24 hours 50.272 minutes (1490.272 minutes). **tier**- another name for a rick, a unit of firewood volume equal to 1/3 cord.
**tierce**- an old English unit of volume, equal to 1/3 butt
or 42 United States gallons. The tierce is almost exactly 159 liters. The
name of the unit is French; it is derived from the Latin
*tertius*meaning 1/3. The tierce is identical to the petroleum barrel [2]. **timber**- a traditional unit of quantity for furs equal to 2 score or 40. This unit, which persisted at least into the nineteenth century, originated because furs were shipped in bundles of 40 pressed between two boards or timbers.
**time**- statements of the time of day are made in various ways in different countries. To avoid confusion in international communications, the International Organization for Standardization (ISO) established (in 1986) International Standard 8601 for representation of dates and times. The ISO 8601 format for time statements is HH:MM, or HH:MM:SS, where HH is the hour number (00 to 23), MM is the minute number (00 to 59), and SS is the second number (00 to 59, except on those very rare occasions where a "leap second" 60 has been introduced as described under day [2]) . Decimal fractions of the second can be added, as in 07:14:23.625. Colons are specified as the separators in the representation (if separators are used at all; HHMMSS is also acceptable). The times called 1:23 am and 1:23 pm in U.S. practice are represented as 01:23 and 13:23, respectively. Midnight can be represented as 00:00 of the new day or as 24:00 of the day ending. For additional information, see ISO's Date and Time Format FAQ.
**time zone [1]**- a unit representing the difference in time between a given location and Universal Time. Under ISO 8601 (see previous entry), the difference is positive if the local time is later than Universal Time and negative if it is earlier; this means that time zones are generally positive in the Eastern Hemisphere and negative in the Western Hemisphere. U.S. Eastern Standard Time is time zone -05, while Pacific Standard Time is time zone -08. For some areas, the difference between local time and Universal Time is not an exact number of hours; these time zones are specified by giving the time difference in hours and minutes. For example, Newfoundland Standard Time is time zone -03:30 or -0330. The time zone is added after a time: for example, 13:23-08 represents 1:23 pm U.S. Pacific Standard Time and is equivalent to 21:23 Universal Time. The same instant is 22:23 in Central European Time, represented by 22:23+01. The traditional acronyms for time zones, such as EST for Eastern Standard Time, may be confusing or meaningless, especially outside their country of origin, so they should not be used in international communications. Worldtimezone.com and thetimenow.com have up-to-the-moment time and time zone information for the entire world.
**time zone [2]**- an informal unit used to express differences in longitude between two places on the Earth. On the average, a time zone spans 15° of longitude. Of course, actual time zones have irregular boundaries, so this is only approximate. At the latitudes of the continental U.S., time zones average about 800 miles wide.
**tin**- an informal unit of volume equal to approximately 8 Imperial fluid ounces (227 milliliters, or roughly the same as a U.S. cup). This is the volume of an English 50-cigarette tin, formerly common as a kitchen unit in North Africa and the Middle East. It is still seen in books of recipes from that area.
**tithe**- a traditional unit of proportion equal to 1/10. The word "tithe" is an old one, drawn directly from the Anglo-Saxon word for a tenth.
**tithing**- an old English unit of land area equal to 1/10 hundred or 10 hides. Very roughly, the tithing was about 12 acres or a little less than 5 hectares.
**TMC ft**- an abbreviation for "thousand million cubic feet," commonly used in water management in India. One TMC ft is equivalent to about 28.317 million cubic meters or 22 956.8 acre feet. One TMC ft/day is about 11 574 cubic feet per second or 327.74 cubic meters per second.
**TME**- a German abbreviation for Technische Mass Einheit (engineering mass unit).
One TME is the mass accelerated at 1 m/s
^{2}by a force of 1 kgf. One TME is equal to 9.80665 kilograms or 21.6200 pounds. This unit is sometimes called the**metric slug**in English, since the slug is defined in a similar way in the English system. The TME is also called the hyl. **to**- a traditional Japanese unit of volume. The to equals 10 sho, which is about 18.039 liters, 3.968 British Imperial gallons, or 4.765 U.S. liquid gallons.
**tod**- a traditional unit of weight equal to 2 stone or 1 quarter. The tod is thus equivalent to 28 pounds or about 12.7 kilograms. "Tod" is an old German word meaning a load.
**toe**- a symbol for
**tonne of oil equivalent**, a unit of energy used in the international energy industry. 1 toe represents the energy available from burning approximately one tonne (metric ton) of crude oil; this is defined by the International Energy Agency to be exactly 10^{7}kilocalories, equivalent to approximately 7.4 barrels of oil, 1270 cubic meters of natural gas, or 1.4 tonnes of coal. 1 toe is also equivalent to 41.868 gigajoules (GJ), 39.683 million Btu (MM Btu) or dekatherms, or 11.630 megawatt hours (MWh). **tog**- a metric unit used to describe the insulating properties of cloth. If the
flow of heat through the cloth is 1 watt per
square meter, then the insulating value in togs is 10 times the temperature
difference, in Celsius degrees (or kelvins),
between the two sides of the cloth. This makes the tog equal to exactly 0.1
m
^{2}K/W or about 0.645 clo. "Togs," now a slang word in English, is derived from an old Dutch/North German word*tuig*or*tög*meaning "the clothes you're wearing." **toise**- a traditional French unit of distance comparable to the British fathom.
Like the fathom, the toise originally represented the distance between the
fingertips of a man with outstretched arms. Sometimes claimed to have been introduced by Charlemagne in
790, the toise is such an ancient unit that
*toiser*has become a verb meaning "to measure" or "to size up." The toise equals six*pieds*(French feet). Feet of different lengths were used in France, but based on the 18th century Paris*pied*the toise equals 6.395 (English) feet or 1.949 meters. This unit was widely used in the 19th century and hasn't died out entirely today. Note that the French have a second fathom-size unit, the brasse, equal to 5 pieds (about 1.624 meters or 5.328 English feet). The brasse was the unit commonly used at sea, while the toise was used on land. **tola**- a traditional unit of weight in India and South Asia. Like the seer and the maund, the tola varied considerably from one area to another. The official size in British India was 180 grains, which is about 0.4114 ounce (avoirdupois) or 11.664 grams. There are 80 tolas in a seer, 3200 in a maund.
**tolerance unit**- a unit of relative distance used by engineers when they put round pegs in round holes. Whenever a shaft passes through a hole, a small distance (the tolerance) must be left so that the shaft will be able to turn. If the diameter of a shaft is D millimeters, one tolerance unit for that shaft is 0.001D + 0.45(cube root of D). The cube root term allows proportionately more tolerance for smaller shafts. The actual tolerance can then be stated as a multiple of the tolerance unit.
**toman**- a historic unit of quantity equal to 10 000. The word is of ancient Persian origin; it survives in several languages of western Asia and is used today in Turkey and Iran.
**ton (tn or T or t) [1]**- a traditional unit of weight equal to 20 hundredweight.
In the United States and Canada, there are 100 pounds
in the hundredweight and exactly 2000 pounds (907.185 kilograms) in the ton.
In Britain, there are 112 pounds in the hundredweight and 2240 pounds (1016.047
kilograms) in the ton. To distinguish between the two units, the British ton
is called the
**long ton**and the American one is the**short ton**. In old England, a "tun" was a large cask used to store wine. Because these tuns were of standard size, more or less, the tun came to represent both a volume unit, indicating the capacity of a cask, and also a weight unit, indicating the weight of a cask when it was full. The best symbol to use for this unit is**tn**. In the U.S. mining industry,**T**is used to distinguish the traditional ton from the metric ton, but T is the SI symbol for the tesla. The symbol**t**, traditionally used for the long or short ton, is now reserved for the metric ton. **ton (t) [2]**- a metric unit of mass, equal to 1000 kilograms, or approximately 2204.623
pounds avoirdupois. This
**metric ton**is a bit smaller than the British long ton. The metric ton is now known officially as the**tonne**(see below). **ton (RT or rT) [3]**- a unit used traditionally to measure the cargo capacity of a merchant ship.
During the Middle Ages, merchant ships were rated by the number of tuns of
wine they could carry. Today the merchant marine ton is defined to be exactly
100 cubic feet, or approximately 2.8316 cubic meters. This unit is often called
the
**register ton**, since it is recorded in official registers of ships. The symbol**RT**seems to be in wide use for this unit, but it is also used for the refrigeration ton (definition [7] below). **ton (DT or dT) [4]**- a unit of volume used traditionally to measure the "displacement" of ships,
especially warships. One way to describe the size of a ship is to state the
volume of sea water it displaces when it is afloat: in other words, the volume
of that part of the ship below the waterline. The actual weight of sea water
varies somewhat according to its temperature and how salty it is, but for
this purpose it has been agreed that a long ton of sea water occupies about
35 cubic feet. Accordingly the
**displacement ton**is defined to be exactly 35 cubic feet, or approximately 0.9911 cubic meter. Since this is a much smaller unit than the register ton, warships have a much higher "tonnage" than merchant ships of approximately the same dimensions. The symbol**DT**is recommended for this unit. **ton (FT) [5]**- a traditional unit of volume used for measuring the cargo of a ship, truck,
train, or other freight carrier. This
**freight ton**is exactly 40 cubic feet, or approximately 1.13267 cubic meters. However, the term "freight ton" is also being used to mean a metric ton of freight, volume not specified. Perhaps because of this confusion, the 40 cubic foot unit is often called the**measurement ton (MTON)**. But the confusion seems impossible to dispel; some shippers are now using "measurement ton" to mean a metric ton of freight. To further complicate the situation, the freight ton is also called the**U.S. shipping ton**; the**British shipping ton**is 5% larger at 42 cubic feet (1.1893 cubic meters). Yet another name is the**cubic ton**. **ton (tn or T) [6]**- a unit of energy used for measuring the energy of an explosion, especially a nuclear explosion. In this usage, one ton is supposed to be the amount of energy released by the explosion of one short ton of TNT. This is defined in the U.S. to equal exactly 4.184 gigajoules (GJ) or roughly 4 million Btu.
**ton (RT) [7]**- a unit of power used in refrigeration engineering. One ton of refrigeration
is intended to be the power required to freeze one short ton of water at 0°C
in 24 hours. This is assumed to be exactly 12 000 Btu
per hour (Btu/h or "Btuh"), which is equivalent to 200 Btu/min, 3.516 853
kilowatts, 4.7162 horsepower,
or 0.8396 (kilogram) Calorie per second (Cal/s).
The symbol
**RT**seems to be in wide use for this unit, but it is also used for the register ton (definition [3] above). **ton [8]**- British slang for 100, especially the sum of 100 pounds (currency), a speed of 100 miles per hour, or a score of 100 in darts or cricket. The origin of this usage is not clear.
**ton mile (tn·mi or tmi)**- a traditional unit of freight transportation, equal to the transportation of one ton for a distance of one mile. The ton mile is equal almost exactly to 1.46 tonne kilometers (1.459 97 to be more exact).
**ton of force (tnf or tn)**- a traditional unit of force, equal to 2000 pounds of force (about 8.8964 kilonewtons) in the U.S. and 2240 pounds of force (about 9.9640 kilonewtons) in Britain.
**ton per square inch (tnf/in**^{2}**or tsi)**- a unit of pressure traditionally used in engineering. In the U.S., 1 tsi = 2000 pounds per square inch or about 13.790 megapascals (MPa). In Britain, 1 tsi = 2240 pounds per square inch or about 15.444 megapascals.
**tønde**- the Danish word for "barrel," traditionally used as a unit of volume equal
to 144 pots or about 139 liters. The tønde
holds 30.6 British Imperial gallons, making
it comparable to the barrel Similarly, one
of the meanings of the German word
*Tonne*is "barrel." **tønde land**- a traditional Danish unit of land area, originally the area that could be planted with a tønde of seed. The tønde land equals 14 000 square alen, which is about 5516 square meters, 0.5516 hectare, or 1.363 acre.
**tone**- another name for the step [2] as a musical unit describing the ratio in frequency between notes.
**tonelada**- a traditional weight unit of Spain and Portugal corresponding to the English and metric ton. The traditional Spanish tonelada equals 2000 libras or about 2028 pounds (919.9 kilograms). The Portuguese tonelada, however, equals 1728 Portuguese libras or arratels; this is about 1748.6 pounds or 793.15 kilograms.
**tonne (t)**- a metric unit of mass equal to 1000 kilograms or approximately 2204.623
pounds avoirdupois. The SI uses this French spelling
for the metric ton (see ton [2] above) to distinguish it clearly from
the long and short tons of customary English usage. Large masses are often
stated as multiples of the tonne, although technically the SI requires that
masses be stated as multiples of the gram. Thus a mass of 10
^{3}tonnes = 10^{6}kg = 10^{9}g is often called 1 kilotonne (kt) instead of 1 gigagram. In the United States, the Department of Commerce recommends that the tonne be called the**metric ton**. **tonne kilometer (t·km or tkm)**- the metric unit of freight transportation, equal to the transportation of one tonne (metric ton) for a distance of one kilometer. The tonne kilometer is equal to almost exactly 0.685 ton miles (0.684 944 ton miles to be more exact.
**tonneau**- the traditional French ton, equal to 2000 livres or about 979 kilograms (1.079 U.S. ton). The tonneau was also used as a measure of volume equal to 42 cubic pieds (50.84 cubic feet, or about 1440 liters). In the wine trade, the tonneau was a shipment of 100 cases, or 1200 bottles (about 900 liters of wine).
**tonnes per hectare (t/ha)**- a common metric unit measuring crop yields. One tonne per hectare is equal to about 0.446 090 U.S. (short) tons per acre.
**TORCON**- an acronym for Tornado Condition Index, a unit devised by the Weather Channel in the U.S. to measure how likely it is that a tornado will form in the vicinity of a location. TORCON
*n*means that (for the duration of the forecast) the probability is*n*·10% that a tornado will occur within 50 miles (about 80 km) from the location. **Torino number**- an arbitrary scale adopted in 1999 to express the likelihood that an asteroid or comet might collide with the Earth causing damage. Named for Torino (Turin), Italy, site of a June 1999 conference on near-Earth objects, the scale is intended to convey accurately the appropriate level of concern caused by newly-discovered objects, thus discouraging sensational press reports. The scale ranges from 0 (object certain to miss the Earth or too small to cause significant damage) to 10 (object certain to hit the Earth and large enough to cause catastrophic damage and global climate disruption). Wikipedia has a description of the scale. See also Palermo scale.
**torr (Torr)**- a non-metric unit of pressure equal to exactly 1/760 atmosphere, about 1.333 22 millibars, 133.322 pascals, or 0.019 337 pound per square inch (psi). The pressure of 1 atmosphere is almost exactly equivalent to the pressure of a column of 760 millimeters of mercury in a mercury barometer. As a result, 1 torr is the same thing as 1 mmHg within 1 part per million. In engineering, the pressures of near-vacuums are often stated in torr. The unit is named for Evangelista Torricelli (1608-1647), the Italian scientist who invented the barometer.
**tot**- a unit of volume for liquor. Generally the term is used informally, with no fixed definition. However, in British pubs the usual understanding is that a tot is 1/6 gill; this is equivalent to 5/6 Imperial fluid ounce or about 23.7 milliliters.
**tour**- another name for a shift in the oil industry. The unit is pronounced "tower." A tour, like a shift, is usually 8 hours in length.
**tovar**- a traditional weight unit of Bulgaria, equal to 100 oka; this is roughly 280 pounds or 128 kilograms.
**tower weights**- the weight system used as the basis for English coinage during the medieval era; it is named for the Tower of London, where the Royal Mint was located. The system was based on the tower pound of 5400 grains (about 0.7714 avoirdupois pounds or 349.91 grams). The pound was divided into 12 ounces, and each ounce contained 20 pennyweight, making the pound equal to 240 pennyweight. (This structure was later echoed, in a reversed way, by the traditional English monetary system, in which the pound was divided into 20 shillings and each shilling into 12 pence.) In 1527, Henry VIII abolished the tower pound in favor of the slightly larger troy pound (see troy weights, below).
**township (twp)**- a traditional unit of area in the U.S. and western Canada. In the U.S. this unit equals 36 sections, or the area of a square nominally 6 miles on a side (about 93.24 square kilometers). Like the section, it is used by the U.S. Public Land Survey System, which applies to most of the country except for the original 13 states, Alaska, and Hawaii. In the four western provinces of Canada, the similar Dominion Lands Survey System used townships made of 36 square sections plus a road allowance between them. From 1871 to 1881 this allowance was 1.5 chains (99 feet), making the township nominally 6.1125 miles on a side (37.36 square miles or 96.77 square kilometers); it was then reduced to 1 chain with some roads removed, making the townships nominally 6.0375 by 6.075 miles (36.68 square miles or 95.00 square kilometers). In both countries the actual size varied slightly due to the curvature of the Earth. See section.
**toz**- a symbol for the troy ounce [2], the traditional unit for weighing precious metals.
**tpi**- an abbreviation for "threads per inch," a unit commonly used to describe screws and other items with similar threads.
**traffic unit (TU)**- an alternate name for the erlang.
**tray**- an informal unit of volume used for berries in U.S. retail produce markets. Berries are typically stocked in corrugated cardboard trays holding six containers of one (dry) quart each, a total volume of about 6.6 liters.
**trey**- an old English word for three, derived from the old French
*treis*(now spelled*trois*). The word survives as the name for a three-spot showing in dice or a three card in card games. **triad**- a unit of quantity equal to 3.
**triennial**- an adjective meaning once every 3 years. Not to be confused with terannual (3 times per year).
**triennium**- a traditional unit of time equal to 3 years.
**trilliard**- a unit of quantity equal to 10
^{21}, which is one sextillion in American terminology or 1000 trillion in traditional British terminology. The name is coined to parallel*milliard*, which has long been a name for 1000 million. **trimester**- a unit of time equal to 3 months or 1/4 year; another name for the quarter [2]. At certain U.S. colleges a trimester is an academic term roughly 14 weeks long. In reckoning the length of human pregnancies, a trimester is a unit of 14 weeks, with the first trimester beginning on the first day of the last menstrual period.
**trio**- a traditional unit of quantity equal to 3. The word is Italian. It originated in music during the Baroque era (early 1700's) in its meaning as a composition in three parts.
**triple, triplet**- a group of 3 items, especially 3 identical items; the word
**triplet**is also used for one member of such a group. Mathematicians prefer**triple**for an ordered string of three elements. **triumvirate**- a unit of quantity equal to 3. The name comes from the Latin
*trium virum*, "of three men," and was first used to describe the governing alliance of Julius Caesar with Pompey the Great and Marcus Licinius Crassus in 60 BCE. **troland (Td)**- a unit used in photometry to measure the amount of light reaching the retina
of the eye. The "retinal illuminance" in trolands is equal to the luminance
of the light shining on the eye, measured in candelas per square meter (cd/m
^{2}), multiplied by the area of the pupil open to the light, in square millimeters (mm^{2}). This calculation makes the troland technically equal to the microcandela (µcd), a unit of luminous intensity, but retinal illuminance is a physical quantity different from luminous intensity. The unit is named for the American experimental psychologist Leonard T. Troland (1889-?). **troy weights**- a traditional English weight system of great antiquity, apparently in use since long before the Norman conquest of 1066. The system is believed to be named for the French market town of Troyes, where English merchants traded at least as early as the time of Charlemagne (early ninth century). The system is based on the troy pound [2] of 5760 grains. The pound was divided into 12 ounces (of 480 grains), each containing 20 pennyweight, with each pennyweight equal to 24 grains. Apothecaries, however, divided the troy ounce into 8 drams (of 60 grains), each containing 3 scruples, with each scruple equal to 20 grains. The origin of the troy system is not clear, but a number of scholars believe the dram corresponds to the denarius, a Roman coin that weighed about 60 English grains and (when used as a weight) was also divided into 3 scruples. The troy system was always the theoretical basis of the traditional English monetary system, in which there were 12 pence (pennies) to the shilling and 20 shillings to the pound. However, in medieval England pennies did not actually weigh a troy pennyweight, because they were made using the tower weight system (see above) and thus weighed 22.5 grains instead of 24. In 1527, Henry VIII abolished the tower pound and made the troy system official for coinage; thereafter silver shillings weighed exactly 0.6 troy ounce. The smaller troy weights continued in common use in pharmacy and monetary affairs into the early twentieth century, but the troy pound was abolished in 1878 to avoid any commercial confusion with the avoirdupois pound. The troy system is nearly obsolete today, but the prices of precious metals are still quoted by the troy ounce.
**trug**- a basket having a shallow, boat-like shape. Invented in Sussex, England, trugs come in many sizes; small ones are often used by gardeners to carry cut flowers. In the U.S., a trug of grain was formerly considered to equal exactly 2/3 U.S. bushel (23.493 liters).
**truss**- a traditional weight unit, generally equal to 4 stone or 56 pounds (about 25.4 kilograms). The truss was used primarily for measuring hay. In 1795, Parliament specified that a truss of hay should equal 56 pounds for old hay or 60 pounds (about 27.2 kilograms) for new hay.
**tsubo**- an informal unit of area used in Japan to measure area inside buildings. One tsubo is an area of about 3.3 square meters (3.95 square yards), an area that can be covered neatly by two traditional tatami mats. See tatami, above.
**t'sun**- see cùn.
**tu**- a traditional Chinese unit of distance equal to 250 li. Using the late nineteenth century definition of the li as 2115 feet, the tu was equal to 100.14 miles or 161.13 kilometers.
**tub [1]**- an old English unit of mass or weight for butter, equal to 84 pounds (38.10 kilograms). In Newfoundland, a tub of coal was formerly equal to 100 pounds.
**tub [2]**- a wide, low container for liquids, generally not of any standard size. During the Prohibition era in the U.S. (1920-1933) liquor smugglers often carried their cargo in tubs holding 4 gallons (15.14 liters). In this they were following an old English tradition dating back at least to tea smugglers of the eighteenth century, who used similar 4-gallon tubs. There are some cases in which the tub has had a legal definition. In Newfoundland, for example, a tub of herring held 16 Imperial gallons (72.74 liters) and a tub of salt held 18 Imperial gallons (81.83 liters).
**tumbler**- another name for the U.S. cup.
**tumblerful**- another name for the breakfast cup, a unit of volume used in British food recipes.
**tun**- a unit of volume used for wine and other liquids. "Tun" is an old French word for a large cask used in shipping wine. Tuns of various sizes were used throughout the Middle Ages. More recently the tun has been regarded as equal to 2 butts or 252 U. S. gallons; this is equivalent to 33.6875 cubic feet or about 953.93 liters. See ton [3] and tonneau, above.
**tunnland**- a traditional unit of land area in Sweden. The tunnland is equal to 56 000
square Stockholm feet (
*kvadratfot*); this is equivalent to 4936.4 square meters, 0.493 64 hectare, or about 1.220 English acre. The tunnland was divided into 32 kappland. Like the Danish tønde land (see above), this unit originated as the area that could be planted with one*tunna*(barrel) of seed. **-tuple**- a suffix added to a number by mathematicians to create units of quantity;
a 7-tuple, for example, contains 7 objects, listed in a specified order. The
suffix has the same Latin root as
**-ply**, meaning -fold. **turbidity unit**- see NTU.
**turn**- another name for a revolution, that is, a unit of angle measure equal to 360°.
**tw**- symbol for the talangwah (see above), a Thai unit of area equal to 4 square meters.
**Twaddle scale**- a specific gravity scale; see degree Twaddle.
**twain**- an old word for the number two, derived from the Anglo-Saxon
*twegen*. The American author Samuel Clemens (1835-1910), who had been a riverboat pilot on the Mississippi in his youth, took his literary name from a traditional riverboat phrase "mark twain", meaning "exactly two" fathoms of water. This was the minimum depth needed for the boats to operate safely without running aground. **twelfth**- a unit used in music to describe the ratio in frequency between notes. Two
notes differ by a twelfth if the higher note has frequency exactly 3 times
the frequency of the lower one. On the standard 12-tone scale, the perfect
twelfth is approximated as 19 half steps,
corresponding to a frequency ratio of 2
^{19/12}= 2.9966. **twelvemonth**- an old English name for a year.
**twip**- a unit of distance used in computer graphics for high-resolution control of the elements of an image. One twip is equal to 1/1440 inch, about 17.639 micrometers, or 0.070 556 kyu. "Twip" is an acronym for "twentieth of a point," which is accurate if the point [2] is interpreted as being exactly 1/72 inch.
**tynnyrinala**- Finnish name for the tunnland (see above).
**typp**- abbreviation for thousand yards per pound, a unit formerly used to describe
yarn density. 1 typp equals about 2.015 907 meters per gram, and
*n*typp corresponds to a yarn density of 2.015 907/*n*tex (see above).

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October 2, 2000