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Fahrenheit
see "degree Fahrenheit".
fall
a traditional unit of distance equal to 6 ells. The fall was used in land measurement somewhat like the rod. Measurements in falls were often made with a rope 6 ells long. The distance falling under the rope was called a fall. The fall was used mostly in Scotland, where its traditional length was 6 Scots ells or about 18.6 English feet (5.67 meters).
fanega
a Spanish unit of volume, mostly for dry goods. About 55.50 liters or 1.960 cubic feet (1.575 U.S. bushels. Very similar units were used in Portugal and in most of the Latin American countries. In Chile and Argentina, however, a much larger fanega, roughly 2.5 U.S. bushels, was customary.
fanega or fanegada
a traditional unit of land area in Spain and in some Latin American countries.  0.643 hectare (1.59 acres). After the introduction of the metric system it became customary to refer to an area 80 meters square as a fanega. This unit is not legal but is still used informally in some parts of Spain.
farad (F)
the SI unit of electric capacitance. A pair of conductors separated by an insulator can store a much larger charge than an isolated conductor can store. The better the insulator, the larger the charge that the conductors can hold. This property of a circuit is called capacitance, and it is measured in farads. One farad is defined as the ability to store one coulomb of charge per volt of potential difference between the two conductors. The unit is named for the British physicist Michael Faraday (1791-1867), who was known for his work in electricity and electrochemistry.
faraday (Fd)
a unit of electric charge. About 96.4853 kilocoulombs or 26.8015 ampere hours, known as Faraday's constant.  it was adopted as a convenient unit for measuring the charges used in electrolysis. One faraday is equal to the product of Avogadro's number and the charge on a single electron.  
fathom (fth or fath)
a traditional unit of distance equal to 2 yards or 6 feet (approximately 1.829 meters). The word comes from the Old English fthm, meaning "outstretched arms", because a fathom is the distance between a man's outstretched fingertips. 
fat half, fat quarter
terms used in fabric retailing to indicate that an order is cut only half the width of the bolt. To cut a fat half, a full yard (36 x 44) is cut in half parallel to the edges of the bolt, that is, a fat half is 36 inches by 22 inches. Similarly, a fat quarter is 1/2 yard by half the width of the bolt (18 inches by 22 inches, in our example).
fatt
a traditional unit of volume for grain, generally equal to 9 bushels or 1/4 chalder. This would be about 317 liters based on the traditional grain bushel now used in the U.S. or about 327 liters based on the British imperial bushel.
FCC unit
a U.S. measure of purity and effectiveness for chemical substances added to foods. FCC stands for Food Chemical Codex, a code of standards prepared by the U.S. Institute of Medicine for the U.S. Food and Drug Administration. FCC units provide a way to judge the relative effectiveness of different preparations, no matter what their weights.
feddan
an Egyptian unit of land area, formerly used throughout the Middle East and North Africa. about 0.42 hectare or 1.038 acre.
femto- (f-)
a metric prefix standing for 10-15 (one quadrillionth). The prefix was coined from femten, the word for fifteen in Danish and Norwegian.
fermi (fm or f)
a metric unit of distance formerly used in atomic physics. One fermi equals 10-15 meter, or 1 femtometer. The unit is named for Enrico Fermi (1901-1954), the Italian-American physicist who built the first nuclear reactor. 
FEU
a unit of cargo capacity, especially for container ships. These ships carry cargo in standard metal boxes, called containers, that can be transferred easily to trains or trucks. FEU is an abbreviation for "forty-foot equivalent unit." One FEU represents the cargo capacity of a standard container 40 feet long, 8 feet wide,  a little over 8 feet high. One FEU equals roughly 25 register tons or 72 cubic meters.
field box
a unit of volume in the U.S. citrus fruit industry, equal to 10 boxes or 16 bushels (0.5638 cubic meter).
fifth
a traditional U.S. unit of liquid volume equal to 4/5 quart, which is the same as 1/5 gallon. The fifth contains exactly 46.2 cubic inches, or about 757.084 milliliters. This unit is an American version of the traditional bottle. a unit used in music to describe the ratio in frequency between notes. Two notes differ by a fifth if the higher note has frequency exactly 3/2 times the frequency of the lower one.
fineness (fine)
a unit of proportion equal to 1/1000. This unit is used to express the purity of alloys of gold or other precious metals. A statement that a gold bar is "999 fine" means that the bar contains at most 0.1% other metals.
finger
a traditional unit of distance equal to 2 nails or 4.5 inches (11.43 centimeters). This unit represents the length of the middle finger, from the tip to the joint where the finger is attached to the hand. a name for the digit, a unit of distance equal to 3/4 inch (19.05 mm). The finger-width was a common unit of measurement in Anglo-Saxon England. An informal measure.
fiscal year
a unit of one year used for budgeting or accounting. A fiscal year has the same length as an ordinary year, but its starting date may be different. In the U.S., for example, the federal government's fiscal year starts on October 1. Generally, fiscal year ends in ordinary year, so the U.S. federal fiscal year 2005 begins October 1, 2004 and ends September 30, 2005.
fist
an informal unit of distance equal approximately to the hand (10 centimeters or 4 inches).  an informal unit of angle measure equal to 10, which is the approximate angle subtended by a clenched fist held at arm's length.
fistmele
a traditional unit of distance equal to the width of a clenched fist with the thumb extended. about 6.5 inches or 16.5 centimeters.
flask
an old name for a bottle, now specialized to mean a narrow, flattened container for alcoholic beverages. a commercial unit of weight formerly used to measure liquid mercury (or quicksilver). The flask was equal to 76.5 pounds avoirdupois, which is equivalent to almost exactly 34.7 kilograms.
flat
an informal unit of angle measure equal to 1/6 turn or 60. Hex nuts have 6 flat sides; to turn the nut "one flat" is to turn it 1/6 revolution.
flock
an old English unit of quantity equal to 2 score or 40.
floor
another name for a story as a unit of height for buildings. 
flops
a unit of computing power equal to one floating point operation per second. In computer science, there is a distinction between fixed point numbers (fixed number of decimal places) and floating point numbers (stored with as many digits as the computer's design allows). A floating point operation is an addition or subtraction of two floating point numbers. The power of supercomputers is being measured in teraflops: trillions of floating point operations per second.
fluid dram or fluidram
a unit of volume in the traditional apothecary system, equal to 1/8 fluid ounce. This unit is usually called the fluid dram or fluidram to avoid confusion with the weight dram. The U. S. fluid dram contains about 0.225 586 cubic inches or 3.696 691 milliliters. The British fluid dram is about 0.216 734 cubic inches or 3.551 633 milliliters.
fluid ounce (fl oz)
a traditional unit of liquid volume, called the fluid ounce to avoid confusion with the weight ounce. In the U. S. customary system there are 16 fluid ounces in a pint, 1.804 687 cubic inches or 29.573 531 milliliters. 
flux unit (fu)
a former name for the jansky, a unit used in astronomy to measure the strength of a radio signal received from an object in the sky.
fod
the traditional Danish foot, equal to about 12.365 inches or 31.41 centimeters.
folio
a folio is 100 words in US; in Britain it is 72 or 90 words, depending on the type of document.
food calorie
the large or kilogram Calorie; see calorie.
foot (ft or ')
The traditional unit of distance.  An ancient unit based on the length of actual feet, is about 25 centimeters (9.8 inches). This unit was replaced in early civilizations of the Middle East by a longer foot, roughly 30 centimeters or the size of the modern unit:

1 foot = 3 hands = 4 palms = 12 inches (thumb widths) = 16 digits (finger widths).

The modern foot (1/3 yard or about 30.5 centimeters) appeared after the Norman conquest of 1066. It may be an innovation of Henry I, who reigned from 1100 to 1135. Later in the 1100s a foot of modern length, the "foot of St. Paul's," was based on a column of St. Paul's Church in London, so that everyone could see the length of this new foot. In the U.S., the Metric Act of 1866 defined the foot to equal exactly 1200/3937 meter, or about 30.480 060 96 centimeters.  In 1959, the U.S. National Bureau of Standards redefined the foot to equal exactly 30.48 centimeters (about 0.999 998 survey foot).
footlambert
a unit of luminance, the brightness of a surface. The footlambert describes the luminance of a surface that emits or reflects one lumen per square foot; it is the luminance of a perfectly reflecting surface under an illumination of one footcandle. 
foot of head
a traditional unit of water pressure used in plumbing and hydraulics. "Head" is short for "headwaters". One foot of head is equivalent to a pressure of 0.433 lb/in2, 2.989 kilopascals (kPa), 29.89 millibars (mb), or 0.882 inches of mercury (in Hg).
foot per minute
a traditional unit of velocity or flow rate. One foot per minute equals exactly 30.48 cm/min, 5.08 mm/s, or 0.018 288 miles per hour.
foot per second
a traditional unit of velocity. One foot per second equals exactly 15/22 mile per hour or exactly 1.097 28 kilometers per hour.
foot pound 
a traditional unit of work, equal to the work done by a force of one pound acting through a distance of one foot. This is equivalent to approximately 1.355 818 joule, 1.285 07 x 10-3 Btu, or 0.323 832 (small) calorie.
foot poundal
a unit of work, equal to the work done by a force of one poundal acting through a distance of one foot.  about 0.031 081 foot pound, or 0.042 140 joule.
foot pound per second 
a traditional unit of power equal to about 1.355 818 watt or 0.001 818 horsepower.
foot ton
a traditional unit of work, equal to the work done by a force of one ton acting through a distance of one foot.
force
a measurement of wind velocity based on the Beaufort scale.
fortnight
a traditional English unit of time equal to 2 weeks or 14 days.
fot
the traditional Swedish foot, equal to 11.689 inches or 29.69 centimeters. The fot was divided into 2 kvarter or into 12 tum.
fourth
a unit used in music to describe the ratio in frequency between notes. Two notes differ by a fourth if the higher note has frequency exactly 4/3 times the frequency of the lower one.
f ratio or f number or f stop
a measure of the light-gathering power of the lenses in cameras and telescopes. The f ratio, for example "f/4" or "f4," is the aperture of the lens, "f/4" indicates that the aperture is 1/4 the focal length.  Because of this connection with exposure times, the f ratio is often said to express the "speed" of a lens.
franklin (Fr)
a CGS unit of electric charge equal to 3.3356 x 10-10 coulombs or 1 statcoulomb. The unit honors Benjamin Franklin (1706-1790), one of the leaders of the American Revolution, who was also an early investigator of electricity.
freight ton
originally a unit of volume equal to 40 cubic feet. More recently, another name for the revenue ton.
French (Fg or Ch)
a unit of distance used for measuring the diameters of small tubes such as catheters, fiber optic bundles, etc. One French is equal to 1/3 millimeter (about 13.123 mils).
fuder
a traditional German unit of liquid volume. roughly 238 U.S. gallons. In the Mosel wine region of Germany, a fuder is now a metric unit equal to 10 hectoliters (1 cubic meter, or 264.17 U.S. gallons). In Austria, the traditional fuder was equal to 18.11 hectoliters (478.42 U.S. gallons), twice the size of the German unit. In Belgium today, the fuder (or foudre) is a metric unit equal to 30 hectoliters (792.52 U.S. gallons).
Fujita-Pearson scale (F)
an empirical scale for estimating the wind speed of a tornado from the damage it causes. The scale, developed by the American meteorologists Theodore Fujita and Allen Pearson, ranges from F0 to F5.
funt
a traditional Russian unit of weight or mass corresponding the German pfund. The funt is equal to about 0.9028 pound avoirdupois or 409.5 grams. The plural is funte.
furlong (fur)
a traditional unit of distance. Long before the Norman Conquest in 1066, Saxon farmers in England were measuring distance in rods and furlongs and areas in acres. A furlong equals 40 rods, 10 chains, 220 yards, 660 feet, or 1/8 mile. One furlong is exactly 201.168 meters. The length of horse races is often stated in furlongs.
fu or fuss
the German foot.

 

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