**f or f/**- see f ratio, below.
**F**- see farad or Fujita scale, below.
**face cord**- a unit of volume for firewood. See rick.
**faden**- an alternate name for the klafter, a German unit of distance.
**faggot**- a traditional unit of volume for firewood. A faggot was 3 feet in length and 2 feet in circumference; this is a volume of about 0.955 cubic feet or 27 liters. There are about 134 faggots in a cord.
**Fahrenheit**- see degree Fahrenheit.
**fall [1]**- a traditional unit of distance equal to 6 ells.
The fall was used in land measurement somewhat like the rod
[1]. Measurements in rods were often made with an actual wood pole, while
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). The
**Scots furlong**was equal to 40 falls (226.8 meters) rather than 40 rods, and the**Scots mile**was equal to 320 falls (5952 English feet, 1.127 English mile or 1814.2 meters). After the unification of Scotland and England the fall was reinterpreted to equal 6 English ells (22.5 feet or 6.858 meters). **fall [2]**- a traditional unit of area equal to one square fall [1]. In the
traditional Scots system of measurement, a fall of land equals about 346 square
feet or 32.15 square meters. A traditional
**Scots acre**was equal to 160 falls or about 6150 square English yards (1.27 English acres or 0.514 hectare). In the English system a fall of land is 506.25 square feet, 56.25 square yards, or about 47.03 square meters. **fanega [1]**- a Spanish unit of volume, mostly for dry goods. The word is derived from
an Arabic word
*faniqa*for a large sack. The fanega equals 12 almudes or 48 cuartillos; this is 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 [2] or fanegada**- a traditional unit of land area in Spain and in some Latin American countries. The unit originated as the amount of land that could be planted with a fanega [1] of seed. It varied considerably from one area to another. In 1801 it was standardized in Spain as the area of a square 96 varas (80.2 meters) on a side; this comes to 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. The Central American manzana is a counterpart of this traditional Spanish unit.
**farad (F)**- the SI unit of electric capacitance. Very early
in the study of electricity scientists discovered that 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. This is a natural definition,
but the unit it defines is very large. In practical circuits, capacitance
is often measured in microfarads, nanofarads, or sometimes even in picofarads
(10
^{-12}farad, or trillionths of a farad). 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. In a process called electrolysis, chemists separate
the components of a dissolved chemical compound by passing an electric current
through the compound. The components are deposited at the electrodes, where
the current enters or leaves the solution. The British electrochemist and
physicist Michael Faraday (1791-1867) determined that the same amount of charge
is needed to deposit one mole of any element
or ion of valence one (meaning that each molecule of the ion has either one
too many or one too few electrons). This amount of charge, equal to about
96.4853 kilocoulombs or 26.8015 ampere
hours, became known as Faraday's constant. Later, 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 (see mole)
and the charge (1
*e*) on a single electron. **fardel**- an old English word meaning a fourth part, used sometimes as a unit of land area equal to 1/4 virgate, 1/2 nook, or about 8-10 acres.
**farsakh, farasang**- Arabic and Persian names, respectively, for the unit classically known as the parasang.
**farthing**- an old English word for 1/4, later used as the name of a coin equal in value to 1/4 penny.
**farthingdale [1]**- an older name for the rood [2], a unit of land area equal to 1/4 acre (0.1012 hectare). "Farthingdale," like "fardel," means a fourth part.
**farthingdale [2]**- a fardel (see above).
**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
*fæthm*, meaning "outstretched arms", because a fathom is the distance between a man's outstretched fingertips. This is a generic unit that has been used in many cultures since ancient times. Other versions include the Spanish braza, the French brasse and toise, the German klafter, the Danish**favn**(6.18 feet or 1.88 meters), the Swedish**famn**(5.84 feet or 1.78 meters) and the Japanese ken. In England, the fathom was a common unit during Saxon times, and it continued to be used for many purposes through the medieval era. In fact, the length of the foot may have been defined, early in the twelfth century, specifically to assure that 1 foot = exactly 1/6 fathom. Today the fathom is used almost exclusively at sea, measuring water depth, the length of ships' cables, etc. **fat half, fat quarter**- terms used in fabric retailing to indicate that an order is cut only half the width of the bolt. For example, if a bolt is 44 inches wide, a yard of fabric is a piece one yard long and 44 inches wide, that is, 36 inches by 44 inches. An ordinary half yard of the fabric would be 18 inches by 44 inches. 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 (1 yard by half the width of the bolt, in general). 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.
**FAU**- abbreviation for
**formazin attenuation unit**, a unit of water turbidity. This unit is used to express turbidity measured by a nephelometer that measures directly the fraction of light transmitted through a water sample as compared to the fraction transmitted through a standard preparation of formazin. The procedure is specified by standard ISO 7027 of the International Organization for Standardization. See NTU for additional information on turbidity. **fbm**- a symbol sometimes used for the board foot. The "bm" stands for "board measure."
**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. Perhaps most familiar to consumers is the FCC unit for the enzyme lactase, taken by those who are lactose intolerant. There is no definite conversion between FCC units and milligrams, because different manufacturers prepare their products in different ways. Instead, 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. The feddan equals 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. This was an attractive idea because it preserved the established symbol for 10^{-15}meter (see next entry). **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. There was a need for this unit before the metric prefixes for very small quantities were defined; now distances formerly measured in fermis are stated in femtometers instead. **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, and (usually) a little over 8 feet high. One FEU equals roughly 25 register tons (see ton [3]) or 72 cubic meters.
**fibrin unit (FU)**- a unit of potency for nattokinase, an enzyme that reduces the viscosity of blood and reduces its tendency to clot. The enzyme was discovered in natto, a traditional cheese-like Japanese food made from a fermented soybean mash. The unit is defined in terms of a specific test of a preparation's ability to dissolve fibrin, the protein found in clots; there is no standard equivalence to milligrams.
**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 [1]**- 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.
**fifth [2]**- 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. On the standard 12-tone scale, the perfect
fifth is very closely approximated as 7 half
steps, corresponding to a frequency ratio of 2
^{7/12}= 1.4983. **fillette**- a half bottle of champagne (375 milliliters).
**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. See also per mill.
**finger [1]**- 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.
**finger [2]**- 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. After the 12-inch English foot became established, the finger survived as an informal measure.
**Finsen unit (FU)**- a unit formerly used in medicine to measure the intensity of the ultraviolet
light used in various medical treatments. Ultraviolet radiation of the standard
wavelength 296.7 nanometers has intensity 1 Finsen unit if its energy density
is 10
^{5}watts per square meter. The unit honors the Danish physician Niels Finsen (1860-1904), who received the Nobel prize for medicine in 1903 for his research on the use of light in the treatment of various diseases. **firkin (fir) [1]**- a traditional unit of volume equal to 1/4 barrel
or 1/2 kilderkin. Since barrels are of
various sizes, the capacity of a firkin varies. Based on the standard U.S.
barrel of 31.5 gallons, a firkin would equal 7.875 gallons, 1.05 cubic feet,
or about 29.81 liters. Traditional British barrels and firkins are larger;
in the Imperial system a firkin holds 1.445 cubic feet or 40.91 liters. The
unit is of Dutch origin, and its name is based on the Dutch word
*vier*for four. **firkin (fir) [2]**- a traditional unit of weight for butter and soap, equal to 4 stone or 56 pounds (about 25.40 kilograms).
**firlot**- a traditional Scottish unit of volume equal to 4 Scots pecks. This is about 36.3 liters for wheat, peas, or beans, or about 48.6 liters for oats or barley. The firlot corresponds closely to the U.S. and British bushel.
**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
*n*ends in ordinary year*n*, so the U.S. federal fiscal year 2005 begins October 1, 2004 and ends September 30, 2005. **fist [1]**- an informal unit of distance equal approximately to the hand (10 centimeters or 4 inches). The unit represents the width of a clenched fist.
**fist [2]**- an informal unit of angle measure equal to 10°, which is the approximate angle subtended by a clenched fist held at arm's length (this works for both adults and children). This unit is used by amateur astronomers to estimate angular distances in the sky.
**fistmele**- a traditional unit of distance equal to the width of a clenched fist with the thumb extended. This is about 6.5 inches or 16.5 centimeters, making the fistmele the same as the Saxon shaftment. The unit is used in archery, where it measures the brace height of a bow (the distance from the center of the grip to the bowstring) and also in kayaking, where it measures various critical dimensions of the boat. In both cases, the intention is that if the fist of the actual archer or kayaker is used in establishing the unit, then the bow or the boat will fit that person precisely.
**FIT**- an acronym for "Failure In Time" or "Failure unIT," a unit used to express
the expected failure rates of semiconductors or other electronic components.
One FIT is equal to a rate of one failure per U.S. billion (10
^{9}) hours, that is, 1 FIT = 10^{-9}/h. Since 10^{9}hours equals about 114 thousand years, these failure rates are estimated from accelerated test procedures using various statistical techniques. **flagon**- a traditional unit of liquid volume, generally equal to the wine (or U.S. liquid) gallon (about 3.785 liters). A flagon is a large, narrow-necked pitcher or bottle.
**flask [1]**- an old name for a bottle, now specialized to mean a narrow, flattened container
for alcoholic beverages. Such flasks come in various sizes, often holding
around 6 U.S. fluid ounces or 180 milliliters. The word is very old, appearing
as
*flaska*in Old High German and as*flascon*in Latin. **flask [2]**- a commercial unit of weight formerly used to measure liquid mercury (or quicksilver, as it was called). 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.
**flick (f) (1)**- a unit of spectral radiance used in optical and communications engineering.
Radiance is the power radiated per unit solid angle per unit of emitting surface.
Radiance varies with wavelength, and to measure this variation spectral radiance
is defined is the radiance per unit of wavelength span. The flick is a short
name for the spectral radiance of 1 watt per
steradian per square centimeter of surface
per micrometer of span in wavelength. This is mathematically equivalent to
10
^{10}watts per steradian per cubic meter. In practice, spectral radiance is typically in microflicks. **flick (f) (2)**- a proposed unit of time, intended to help in coordinating the frame rates of various video display devices. The flick is defined to be 1/705 600 000 second or about 1.417 234 nanosecond.
**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. In Britain, the floor above the ground floor is usually called the first floor, so in a building with four stories (or storeys) the top floor is called the third floor. In the U.S., the numbering of floors is most often the same as the numbering of stories.
**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 (which have a fixed number of decimal places) and floating point numbers (which are 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 (Tflops): trillions of floating point operations per second.
**fluid dram or fluidram (fl dr)**- 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. In the British Imperial system, the 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, so each fluid ounce represents 1.804 687 cubic inches or 29.573 531 milliliters. In the British Imperial system there are 20 fluid ounces in an Imperial pint, so each fluid ounce represents about 1.733 871 cubic inches or 28.413 063 milliliters. A U.S. fluid ounce of water weighs just a bit more than one ounce avoirdupois; a British fluid ounce weighs exactly one ounce at a specified temperature and pressure.
**fluid scruple**- a traditional British unit of liquid volume equal to 1/3 fluidram or about 1.1839 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.
**FNU**- abbreviation for
**formazin nephelometric unit**, a unit of water turbidity equivalent to the NTU (nephelometric turbidity unit) used in the U.S. The symbol FNU, specified by the International Organization for Standardization in its standard ISO 7027, is widely used outside the U.S. See NTU. **f number**- see f ratio, below.
**fod**- the traditional Danish foot, equal to about 12.365 inches or 31.41 centimeters.
**foe**- an informal unit of energy introduced by the German-American physicist Hans Bethe (1906-2005) and used by astrophysicists to express the energy
released by supernovas. One foe equals 10
^{51}ergs or 10^{44}joules. The word "foe" is an acronym for [10 to the] fifty-one ergs. This unit is now called the bethe in Bethe's honor. **folio**- in traditional legal practice, a unit of quantity for the words in a legal
document. In the U.S., a folio is 100 words; in Britain it is 72 or 90 words,
depending on the type of document. In the days before mechanical copying,
clerks were paid by the folio to copy legal documents. The word is from the
Latin
*folium*, leaf, and originally referred to a page; apparently the unit once represented the number of words on a page. **food calorie**- the large or kilogram Calorie; see calorie [2].
**foot (ft or ')**- a traditional unit of distance. Almost every culture has used the human
foot as a unit of measurement. The
**natural foot**(*pes naturalis*in Latin), 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, because this longer length was conveniently expressed in terms of other natural units: - 1 foot = 3 hands = 4 palms = 12 inches (thumb widths) = 16 digits (finger widths)
- This unit was used in both Greece and Rome; the Greek foot is estimated
at 30.8 centimeters (12.1 inches) and the Roman foot at 29.6 centimeters (11.7
inches). In northern Europe, however, there was a competing unit known in
Latin as the
*pes manualis*or**manual foot**. This unit was equal to 2 shaftments, and it was measured "by hand," grasping a rod with both hands, thumbs extended and touching. The manual foot is estimated at 33.3 centimeters (13.1 inches). - In England, the Roman foot was replaced after the fall of Rome by the natural foot and the Saxon shaftment (16.5 centimeters). The modern foot (1/3 yard or about 30.5 centimeters) did not appear until 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 inscribed on the base of a column of St. Paul's Church in London, so that everyone could see the length of this new foot. From 1300, at least, to the present day there appears be little or no change in the length of the foot.
- Late in the nineteenth century, after both Britain and the U.S. signed
the Treaty of the Meter, the foot was officially defined in terms of the
new metric standards. 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. This
unit, still used for geodetic surveying in the United States, is now called
the
**survey foot**. In 1959, the U.S. National Bureau of Standards redefined the foot to equal exactly 30.48 centimeters (exactly 0.999 998 survey foot). This definition was also adopted in Britain by the Weights and Measures Act of 1963, so the foot of 30.48 centimeters is now called the**international foot**.

Elsewhere on this page are entries for the Danish**fod**, Swedish**fot**, and German**fuß**. Other foot units include the French pied, Italian and Spanish pie, and Hungarian láb.**(fu**ss) **football field [1]**- a common informal unit of distance in the United States and Canada. Americans aren't quite agreed as to whether the unit is exactly 100 yards (91.44 meters), the distance between the goal lines on an American football field, or 120 yards (109.728 meters), the distance including the two end zones. Canadian football fields are 110 yards (100.58 meters) long between the goal lines and 150 yards (137.16 meters) including the end zones. As a distance equal to the length of an athletic field, this unit is analogous to the classical stade or stadium, although the stade is roughly twice as long as a football field.
**football field [2]**- an informal unit of area in the United States and Canada. Including the end zones, an American football field represents an area of about 1.3223 acres or 0.535 hectare while the Canadian football field has an area of 2.0145 acres or 0.815 hectare.
**footcandle (fc or ftc)**- a traditional unit of illuminance or illumination, defined as the illuminance received by a surface at a distance of one foot from a source of intensity one international candle. The "international candle" was the predecessor of the candela as the standard unit of light intensity. Illuminance is now measured in lux; one footcandle equals 10.764 lux or about 1.0764 milliphot. The unit is also spelled foot-candle or foot candle.
**footlambert (fL, fl or ftL)**- 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. One footlambert equals 3.426 259 candelas
per square meter or about 1.0764 millilambert
(mLb). The unit is also spelled foot-lambert or foot lambert. The one-word
spelling and symbol
**fL**have been endorsed by the Illuminating Engineering Society of North America (IES) and by the Institute of Electrical and Electronics Engineers (IEEE). (Of course, fL is also the SI symbol for the femtoliter.) **foot of head (ft hd)**- a traditional unit of water pressure used in plumbing and hydraulics. "Head"
is short for "headwaters"; it refers to the depth of the water upstream from
the point at which the pressure is measured. One foot of head is equivalent
to a pressure of 0.433 lb/in
^{2}, 2.989 kilopascals (kPa), 29.89 millibars (mb), or 0.882 inches of mercury (in Hg). **foot per minute (ft/min or fpm)**- 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 (ft/s or ft/sec or fps)**- 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 (ft·lbf or ft·lb) [1]**- 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 pound (ft·lbf or ft·lb) [2]**- another name for the traditional unit of torque also known as the pound foot.
**foot poundal**- a unit of work, equal to the work done by a force of one poundal acting through a distance of one foot. One foot poundal is equivalent to about 0.031 081 foot pound, or 0.042 140 joule.
**foot pound per second (ft****·**lbf/s or ft**·**lb/s)- a traditional unit of power equal to about 1.355 818 watt or 0.001 818 horsepower.
**foot ton (ft·tn)**- a traditional unit of work, equal to the work done by a force of one ton acting through a distance of one foot. In the U.S. system, a foot ton equals 2000 foot pounds (2.7116 kilojoules or 2.570 Btu); in the British Imperial system a foot ton equals 2240 foot pounds (3.0370 kilojoules or 2.8786 Btu).
**force**- a measurement of wind velocity based on the Beaufort scale.
**forpet**- Scottish for a 1/4 part, generally meaning 1/4 peck; thus the forpet is another name for a lippie.
**fortnight**- a traditional English unit of time equal to 2 weeks or 14 days. The word, a contraction of "fourteen nights," has been used since at least the 1100s.
**fot**- the traditional Swedish foot, also called the
**Stockholm foot**, equal to 11.689 inches or 29.69 centimeters. The fot was divided into 2**kvarter**or into 12**tum**. **fother**- a traditional English unit of weight for lead. The fother, equal to 30 fotmals (next entry), was always a little smaller than a (long) ton. The original version seems to have been equivalent to 2160 avoirdupois pounds, and the version still being used in the nineteenth century was equal to 19.5 hundredweight or 2184 pounds.
**fotmal**- a traditional English unit of weight for lead. In medieval England the fotmal was equal to 70 "mercantile" pounds, which is equivalent to 72 avoirdupois pounds.
**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. On the standard 12-tone scale, the perfect
fourth is very closely approximated as 5 half
steps, corresponding to a frequency ratio of 2
^{5/12}= 1.3348. **fpm**- a common abbreviation for foot per minute (ft/min), a traditional unit of flow (see above).
**fps**- a common abbreviation for foot per second (ft/s or ft/sec), a traditional unit of speed or flow, or for frames per second, a unit used in video processing.
**frail**- a kind of basket, sometimes used as a commercial unit of weight for fruit. Depending on the item, a frail could equal anywhere from 32 to 75 pounds (15-34 kilograms). In the nineteenth century a frail of raisins was often taken to equal 50 pounds (22.68 kilograms).
**f ratio or f number or f stop (f/ or f)**- 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 (the
effective diameter of the lens, which may be reduced or "stopped down" for
the exposure; see also stop) expressed as a fraction of the focal length
of the lens (the distance from the lens to the point where light is focused).
Thus "f/4" indicates
that the aperture is 1/4 the focal length. In cameras the f ratio is proportional
to the square root of the exposure time, so an f/8 setting requires a four
times ((8/4)
^{2}) longer exposure than an f/4 setting. 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. An electrostatic charge of one franklin exerts a force of one dyne on an equal charge at a distance of one centimeter. 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; see ton [5]. 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). The name and the symbol
Ch refer to the
**Charrière gauge**scale, which is often called the French scale. **frigorie**- a unit of refrigeration formerly used in Europe, equal to 1 (kilogram) Calorie
per hour (4.1868 kilojoules
*calor*, for heat, with*frigor*, for cold. **f stop**- see f ratio, above.
**fsw**- symbol for "feet of seawater," a conventional unit of pressure. 1 fsw = 0.3048 meter of seawater (msw).
**FTE**- abbreviation for "full time equivalent," a unit of the rate of work equivalent to that performed by a employee who holds a regular appointment. In the U.S., the FTE is generally equal to 40 hours per week. The symbol FTE is also used for the amount of work performed at this rate over a set period of time, such as a month (170 hours) or a year (2080 hours).
**fuder**- a traditional German unit of liquid volume. A "fuder" is a cartload. In most of the German states the traditional fuder held about 9 hectoliters (roughly 238 U.S. gallons), making the fuder about the same size as the British tun or the French wine tonneau. 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 scale (EF)**- an empirical scale for estimating the wind speed of a tornado from the damage it causes. The scale, as developed by the American meteorologists Theodore Fujita and Allen Pearson, ranged from F0 to F5. In 2007 the U.S. National Weather Service began using a revised version called the EF (Enhanced Fujita) scale; the designations in the new scale are EF0 to EF5.
**full step**- see step [2].
**funt**- a traditional Russian unit of weight or mass corresponding the German pfund
(which is pronounced "funt"). 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. The word "furlong", from the
Old English
*fuhrlang*, means "the length of a furrow"; it represents the distance a team of oxen could plow without needing a rest. A furlong equals 40 rods [1], which is exactly 10 chains, 220 yards, 660 feet, or 1/8 mile. One furlong is exactly 201.168 meters, so a 200-meter dash covers a distance very close to a furlong. The length of horse races is often stated in furlongs. **fuß or fuss**- the German foot (see foot, above). The length of the fuß varied somewhat;
the Viennese version was equal to 12.444 inches or 31.608 centimeters, while
the
**Rheinfuss**(Rhine foot), used in much of western and northern Germany, was equal to 12.357 inches or 31.387 centimeters. In Bavaria a shorter fuß of about 29 centimeters was used. There's no change in the plural.

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All material in this folder is copyright 2008 by Russ Rowlett and the University of North Carolina at Chapel Hill. Permission is granted for personal use and for use by individual teachers in conducting their own classes. All other rights reserved. You are welcome to make links to this page, but please do not copy the contents of any page in this folder to another site. The material at this site will be updated from time to time.

September 11, 2001; revised December 9, 2008