**g**- a symbol for the average acceleration produced by gravity at the Earth's
surface (sea level). The actual acceleration of gravity varies from place
to place, depending on latitude, altitude, and local geology. The symbol
*g*is often used informally as a unit of acceleration. By agreement among physicists, the**standard acceleration of gravity***g*is defined to be exactly 9.806 65 meters per second per second (m/s_{n}^{2}), or about 32.174 05 feet per second per second. At latitude*p*, a conventional value of the acceleration of gravity at sea level is given by the**International Gravity Formula**,

*g*(*p*)=9.7803267714(1+0.00193185138639sin^{2}(*p*))/√(1-0.0069437999013sin^{2}(*p*)). The variation, caused by the oblateness of the Earth and the accleration we experience due to the rotation of the Earth, is about half a percent, from 9.780 327 m/s^{2}at the Equator to 9.833 421 m/s^{2}at the poles. - The symbol
*g*was used as a unit first in aeronautical and space engineering, where it is important to limit the accelerations experienced by the crew members of aircraft and spaceships: the "*g*forces," as they are called. This use became familiar through the space programs, and now a variety of accelerations are measured in*g*'s. The names**gee**and**grav**is also used for this unit. Note that g is also the symbol for the gram. **G [1]**- informal abbreviation in computer science for 2
^{30}= 1 073 741 824. See also giga- [2] and gibi-. **G [2]**- a symbol for grand, a slang term for 1000.
**Ga**- symbol for one billion (10
^{9}) years. The*a*stands for the Latin*annus*, year. **gage**- alternate spelling for gauge (see below).
**galactic year**- the unit of time in which the Solar System makes one revolution around the center of the Milky Way galaxy. The galactic year is estimated to be about 225 million ordinary years. The age of the Solar System is about 20 galactic years.
**galileo (Gal or gal)**- the CGS unit of acceleration. One galileo is
an acceleration of 1 centimeter per second per second (cm/s
^{2}). This unit is used by geologists, who make careful measurements of local variations in the acceleration of gravity in order to draw conclusions about the geologic structures underlying an area. These variations are typically measured in milligals (mGal). One Gal is approximately 0.001 019 7*g*, where*g*is the acceleration of gravity, so a milligal is a very small acceleration, about 10^{-6}*g*. The name of the unit honors the Italian astronomer and natural philosopher Galileo Galilei (1564-1642), who proved that all objects at the Earth's surface experience the same gravitational acceleration. To avoid confusion with the symbol for the gallon, and to conform to the usual metric style, the symbol for this unit should be Gal rather than gal. **gallon (gal) [1]**- a traditional unit of liquid volume, derived from the Roman
*galeta*, which originally meant a pailful. Gallons of various sizes have been used in Europe ever since Roman times. In the United States, the liquid gallon is legally defined as exactly 231 cubic inches; this is equal to the old English**wine gallon**, which originated in medieval times but was not standardized until 1707, during the reign of Queen Anne. Some scholars believe the wine gallon was originally designed to hold 8 troy pounds of wine. The U. S. gallon holds 4 liquid quarts or exactly 3.785 411 784 liters; a U.S. gallon of water weighs about 8.33 pounds. American colonists were also familiar with the Elizabethan**beer and ale gallon**, which held 282 cubic inches (4.621 liters). **gallon (gal) [2]**- a historic British unit of dry volume still used implicitly in the U.S.
In the U.S., the term "gallon" is not used in dry measure, but if it were
it would be equal to 1/2 peck, or 4 dry quarts,
or 268.8025 cubic inches, or approximately 4.404 884 liters.
This unit is the English
**corn**or**grain gallon**, standardized during the reign of Elizabeth I in the sixteenth century. The earliest official definition of a dry gallon in Britain is a 1303 proclamation of Edward I, where the gallon is defined as the volume of 8 pounds of wheat; the current U.S. "gallon" contains about 7.5 pounds of wheat. Grain gallons have tended to be larger than liquid gallons throughout the history of British units, apparently because they were based on heaped rather than "struck" (leveled) containers. A container in which grain has been heaped above the top will hold as much as 25% more grain, and the traditional corn gallon is in fact 16.4% larger than the wine gallon. **gallon (gal) [3]**- a British Imperial unit of volume larger than either of the American
gallons. The Imperial Weights and Measures Act of 1824 established a new
unit for all volumes, liquid or dry, replacing all the other gallons in previous
use in Britain. The
**Imperial gallon**, designed to contain exactly 10 pounds of distilled water under precisely defined conditions, holds exactly 4.546 09 liters or approximately 277.4194 cubic inches. The Imperial gallon equals 1.20095 U.S. liquid gallons (British wine gallons) or 1.03206 U.S. dry gallons (British corn gallons). **gallon (gal) [4]**- a traditional unit of volume in Scotland equal to 4 Scots quarts. This is almost exactly 3 British Imperial gallons, 3.6 U.S. liquid gallons, or 13.63 liters.
**gallon of gasoline equivalent (GGE)**- a unit of energy established by the U.S. Environmental Prorection Agency to allow comparisons of energy efficiency between various energy sources for motor vehicles. The GGE is equal to 114 000 Btu or 33.41 kilowatt hours; this is a typical energy content for a gallon of gasoline. For each energy source EPA has identified a quantity of that source that delivers one GGE of energy.
**gallon per 100 miles (gal/100 mi)**- a measure of fuel consumption rate for vehicles. In metric countries fuel consumption is stated in liters per 100 kilometers (L/100km); this is the corresponding measure in traditional U.S. units. Since 2013 it has been stated on the window stickers of new vehicles in addition to the more familiar (to Americans) measure of miles per gallon. One gallon per 100 miles is equal to about 2.352 15 liters per 100 kilometers. A consumption rate of
*x*gallons per 100 miles equals exactly 100/*x*miles per gallon. **galopin**- a French name for a small glass of beer, typically 200 milliliters (about 6.76 U.S. fluid ounces).
**gamma (γ) [1]**- a unit of magnetic flux density equal to 10
^{-9}tesla (1 nanotesla) or 10^{-5}oersted (10 µOe). In geophysics, small changes in the Earth's magnetic field are traditionally stated in gammas. The nanotesla (nT) is now recommended for these measurements. **gamma (γ) [2]**- an informal metric unit of mass equal to 1 microgram (µg).
**garnets**- a traditional unit of liquid volume in Russia. The garnets is equivalent to approximately 3.28 liters; this is about 3.47 U.S. quarts or 2.89 British Imperial quarts.
**gas permeation unit (GPU)**- a CGS unit of gas permeance for membranes,
contact lenses, and similar thin materials. Permeance is defined to be
the gas flow rate through the material, per unit of area and per unit of
pressure difference across the material. The unit is equal to 10
^{4}barrer per centimeter, or 10^{-6}cm·s^{-1}·cmHg^{-1}, or, in SI units, 7.5005 x 10^{-16}m·s^{-1}·Pa^{-1}. **gauge (ga) [1]**- a traditional unit measuring the interior diameter of a shotgun barrel. The gauge of a shotgun was the number of lead balls, each of a size just fitting inside the barrel, that were required to make up a pound. In other words, if a lead ball weighing 1/12 pound just fit in the barrel of a shotgun, then it was a 12-gauge shotgun. Today, the internal diameters for each gauge number are read from a table.
**gauge (ga) [2]**- a unit expressing the fineness of a knitted fabric, equal to the number of loops per 1.5 inches (38.1 millimeters). The same unit is also used to express the size of the knitting needles used to create a fabric of that fineness.
**gauge (ga) [3]**- a traditional unit measuring the diameter (or the cross-sectional area)
of a wire. Various wire gauge scales have been used in the U.S. and Britain.
In traditional scales, larger gauge numbers represent thinner wires. (For
very thick wires, repeated zeros are used instead of negative numbers, so
gauges 00, 000, and 0000 represent -1, -2, and -3, respectively.) In the
**American Wire Gauge**(AWG) scale, 0000 gauge represents a wire having a diameter of 0.46 inch and 36 gauge represents a diameter of 0.005 inch (5 mils). Diameters for the other gauges are obtained by geometric interpolation, meaning that the ratio between successive diameters is a constant, except for necessary roundoff. Thus*n*gauge wire has a diameter of .005·92^{((36-}^{n}^{)/39)}inch. The**metric wire gauge**number is equal to the cross-sectional area of the wire in square millimeters. Wikimedia has a table of wire gauge equivalents. **gauge (ga) [4]**- a traditional unit measuring the thickness of sheet metal. Larger gauge numbers represent thinner metal: 10 gauge represents a thickness of 0.1345 inch (3.416 millimeters) and each increase of 1 in the gauge number corresponds to a reduction of about 10% in the thickness. Wikimedia has a table.
**gauge (ga) [5]**- a traditional unit measuring the thickness of plastic film. For this purpose
1 gauge equals 0.01 mil [1] or 10
^{-5}inch (0.254 micrometer). **gauge (ga) [6]**- a traditional unit measuring the thickness of tennis racquet strings. There are two systems in use. In the U.S. system larger gauge numbers indicate thinner strings; in the European system larger gauge numbers indicate thicker strings. Tennis-warehouse.com has a table of U.S. sizes.
**gauss (G or Gs) [1]**- the CGS unit of magnetic flux density. A field
of one gauss exerts, on a current-carrying conductor placed in the field,
a force of 0.1 dyne per ampere
of current per centimeter of conductor. One gauss represents a magnetic flux
of one maxwell per square centimeter of cross-section
perpendicular to the field. In SI units, one gauss
equals 10
^{-4}tesla. The unit is named for the German mathematician and astronomer Karl Friedrich Gauss (1777-1855). **gauss (G or Gs) [2]**- a former name for the CGS unit of magnetic field strength, officially renamed the oersted in 1930.
**gauss (G or Gs) [3]**- the CGS unit of magnetic dipole moment per unit
volume, more commonly written emu/cm
^{3}or emu/cc. In this use the gauss equals 1000 amperes per meter in SI units. **GDU**- a symbol for
**gelatin digesting unit**, used for measuring dosage of bromelain, an enzyme used as a digestive aid and for reduction of pain and inflammation. This unit cannot be converted to a weight unit, because different preparations of the enzyme differ in activity. Bromelain is also measured in milk clotting units (MCU); 1 GDU equals approximately 1.5 MCU. **gear inch**- a traditional unit for measuring the gears of bicycles. In low gears, the pedals are easy to turn but have to be turned very fast to achieve any speed; in high gears the pedals are harder to turn but don't have to be turned fast to achieve high speed. The gear value is computed in gear inches as the diameter of the drive wheel times the size of the front sprocket divided by the size of the rear sprocket, all measurements being in inches. (This is the same as the diameter of the drive wheel times the number of gear teeth on the front sprocket divided by the number of teeth on the rear sprocket.) This is the diameter that the drive wheel would need to have to give the same pedal effort as if the pedals were attached directly to the wheel, as on a child's tricycle. Values range from about 25 gear inches for the low gears on mountain bikes to more than 100 gear inches for the highest gears on racing bikes.
**gee**- an informal name for
*g*, the standard acceleration of gravity (see above). **gee pound**- another name for a slug.
**gemmho**- another name for the micromho or microsiemens (µS), a unit of electrical conductance.
**generation (gen)**- an informal unit of time. Roughly speaking, a generation is the average length of time between the birth of a parent and the birth of the child. This leaves a question, however: should just the father, or just the mother, or both parents be included in the calculation? Various answers to this question, plus a lack of consistent data, have led to a range of estimates for the length of a generation, from about 25 to 35 years. Genealogists tend to use the higher figures, anthropologists the lower ones. There is some research suggesting that the approximate length of the generation in the world today is about 28 years.
**German legal meter (Glm)**- an obsolete unit of distance, longer than a meter by 13.5965 micrometers (less than 1 part in 70 000). This unit was used in surveying in Namibia, a former German colony.
**geographical mile**- another name for the nautical mile,
especially the Admiralty mile (6080 feet or 1853.184 meters). For a (different)
German use of the term
*geographische meile*, see meile. **GeV**- the symbol for one billion (10
^{9}) electronvolts. Thanks to Einstein's equation*E = mc*equating mass wth energy, the GeV can be regarded either as a unit of energy equal to 160.217 646 2 picojoules, or as a unit of mass equal to 1.782 662 x 10^{2}^{-24}gram or 1.073 544 atomic mass unit. **gf**- symbol for gram force (see below under gram weight).
**g/g**- a symbol for "gram per gram", a unit of mass concentration. For example, a concentration of 0.02 g/g, or 2% g/g, means that the substance being measured comprises 2% of the mass of the mixture in which it is found. This symbol is equivalent to the traditional symbol w/w.
**GGE**- a symbol for gallon of gasoline equivalent (see that term above).
**gibi- (Gi-)**- a binary prefix meaning 2
^{30}= 1 073 741 824. This prefix was adopted by the International Electrotechnical Commission in 1998 to replace giga- for binary applications in computer science. In particular a**gibibyte (GiB)**is equal to 2^{30}= 1 073 741 824 bytes. The prefix is a contraction of "gigabinary." **gig**- an informal contraction of the gigabyte (GB) common in computer science, especially as a unit of storage capacity.
**giga- (G-)**- a metric prefix denoting 10
^{9}or one billion (in the American meaning of the word billion). This prefix was coined from the Greek word*gigas*, meaning giant. The Greeks pronounced the word with a two hard g sounds, as in "gig." The prefix is sometimes pronounced with an initial soft g sound, as in "jig." Some dictionaries favor this choice, but the hard g pronunciation seems more common. **gigabecquerel (GBq)**- a unit of radioactivity equal to one billion atomic disintegrations per second or 27.027 millicuries.
**gigabyte (GB)**- a unit of information equal to one billion bytes. This unit is often known informally as a "gig." The gigabyte is often used to mean 2
^{30}= 1 073 741 824 bytes but this use breaks the rules of the SI. The unit equal to 2^{30}bytes should be called the**gibibyte**(see above). **gigaflops (Gflops)**- a unit of computing power equal to one billion (10
^{9}) floating point operations per second. See flops. **gigagram (Gg)**- an SI unit of mass equal to 10
^{9}grams, one million kilograms, or 1 kilotonne (1000 metric tons). This unit is used in scientific contexts, such as the measurement of atmospheric emissions. One gigagram equals about 2.2046 million pounds or 1102.3 U.S. (short) tons. **gigahertz (GHz)**- a unit of frequency equal to 10
^{9}per second, or 1 per nanosecond. Cellular phones and microwave ovens operate with radio waves having frequencies in the gigahertz range. **gigajoule (GJ)**- a metric unit of energy commonly used in the energy industry. One gigajoule equals 947 817 Btu, 277.7778 kilowatt hours, or about 9.48 therms.
**gigaliter (GL or Gl)**- a metric unit of volume equal to 10
^{9}liters or one million cubic meters. This is equivalent to 810.713 acre feet or 35.315 million cubic feet. **gigameter (Gm)**- a metric unit of distance equal to one million kilometers or about 621 371.2 miles. Seldom used, this unit would be a good yardstick for the inner Solar System: the distance from the earth to the sun is about 149.60 gigameters.
**gigaparsec (Gpc)**- a non-metric unit of distance equal to one billion parsecs,
3.2616 billion light years, or 30.856
78 zettameters (30.856 78 x 10
^{21}kilometers). The unit is used in astronomy. **gigapascal (GPa)**- a metric unit of pressure. One gigapascal equals 10 kilobars or approximately 145 038 pounds (72.519 short tons) per square inch. Pressures in this range are common inside the earth and can be produced by various high-energy events.
**gigatonne (Gt)**- a metric unit of mass or weight equal to one billion metric tons (tonnes) or about 2.2046 trillion pounds. This very large unit is used, for example, in discussing the amounts of carbon added to the atmosphere by human activities.
**gigawatt (GW)**- a metric unit of power equal to one billion (10
^{9}) watts or about 1.341 million horsepower. **gigawatt hour (GW·h)**- a metric unit of energy equal to one billion watt hours, 3.6 terajoules, or about 3.412 billion Btu.
**gigayear (Gyr)**- a unit of time equal to one billion years. The international symbol
**Ga**is also used, the "a" being taken from the Latin word*annus*for a year. **gilbert (Gi)**- the CGS unit of magnetomotive force, equal to 10/4pi = 0.795 775 ampere-turns. The unit is named for the English physicist and physician William Gilbert (1540-1603), who published his discoveries on magnetism in 1600.
**gill (gi)**- a traditional unit of volume for liquids, especially wine and other alcoholic
beverages. The gill is 1/4 pint. In the U.
S. customary system, one gill is equal to 1/2 cup,
4 fluid ounces, 7.21875 cubic inches, or about 118.3 milliliters. In the
British Imperial system, the gill equals 5 fluid ounces, 9.023 cubic inches,
or about 142.1 milliliters. The unit is pronounced "jill", with a soft "g" sound.
Its name comes a Latin word
*gillo*for a small wine vessel. **gillion**- an informal alternate name for the number 10
^{9}, called "billion" in America but often called "milliard" in France and "thousand million" in Britain. The reasoning here is that if mega- means a million and tera- a trillion, then giga- should mean a gillion! **gin**- one of several spellings used in English for the jin, the Chinese weight unit also called the catty.
**g/kg**- a symbol for "gram per kilogram", a unit of mass concentration equal to 1 per mill (1 part per thousand). 1 g/kg also equals 0.1% g/g or 0.1% w/w.
**Glasgow coma scale (GCS)**- a scale used in emergency medicine to assess the condition of trauma victims and other partially-conscious patients. The scale range is 3-15; patients with Glasgow scores of 3-8 are usually said to be in a coma. The score is obtained by observing the patient's eye-opening, motor response, and response to commands. Wikipedia has a description of the scale.
**glass [1]**- a unit of time measured by an hourglass or sandglass. At sea, time was traditionally measured with half-hourglasses, making the glass a nautical unit of time equal to 1/2 hour. In this use the glass is another name for the bell.
**glass [2]**- another name for the U.S. cup (236.6 milliliters). Doctors in the U.S. are fond of saying that everyone should drink 8 glasses of water a day, and this is the amount they have in mind for a glass.
**glass [3]**- an informal unit of volume used in Australian pubs. In several states of Australia a glass of beer is usually 200 milliliters, but it is 235 milliliters in Queensland and 285 milliliters in Western Australia.
**glean**- an old English unit of quantity for herrings, equal to 25 fish.
**gm**- an incorrect symbol for the gram (see below). The correct symbol is g. In engineering, "gm" is sometimes used to distinguish "gram mass" from "gram force" (gf). This practice is not recommended.
**GMT**- abbreviation for Greenwich Mean Time, the standard time of longitude 0°.
This meridian of longitude, called the
*prime meridian*, was fixed as the longitude of the Royal Greenwich Observatory in London by the International Meridian Conference of 1884. GMT is five hours later than U.S. Eastern Standard Time. The abbreviation UT (Universal Time) has largely replaced GMT. **gnat's eye**- an idiomatic "unit" of distance; it's common to hear that something is "as small as a gnat's eye." In fact, the eyes of typical gnats tend to have diameters similar in size to a hair's breadth: roughly 100-150 micrometers (0.10-0.15 millimeters).
**go**- a traditional Japanese unit of liquid volume. One go is about 180.39 milliliters, 0.3812 U.S. pint, or 0.3174 British Imperial pint.
**goad**- a traditional unit of distance sometimes used in measuring cloth. One goad is equal to 54 inches or 1.5 yards (1.3716 meters). A goad was originally a spear; later it was a pointed rod used for prodding animals to get a move on. The unit must have originated as the length of such an instrument.
**gon****(**^{g})- another name for the grad, a unit of angle measurement equal to 1/400 circle,
0.01 right angle, 0.9°, or 54'. This name was formerly used in German,
Swedish, and other languages in which the word
*grad*means*degree*. The gon is rarely used today. The name is taken from the ancient Greek word*gonia*, angle. **gong-**- in the Pinyin system for transliterating Chinese, the word
*gong*(preferably with a macron, or long-vowel sign, over the*o*) represents a character whose basic meaning is "public" or "official." This meaning has been extended in recent years to include "metric." Thus a**gongjin**is a metric jin, that is, a kilogram, and a**gongli**is a metric li, or kilometer. **googol**- a unit of quantity equal to 10
^{100}(1 followed by 100 zeroes). The googol was invented by the American mathematician Edward Kasner (1878-1955) in 1938. According to the story, Kasner asked his nephew Milton Sirotta, who was then 8 years old, what name he would give to a really large number, and "googol" was Milton's response. Kasner also defined the**googolplex**, equal to 10^{googol}, that is, 1 followed by a googol of zeroes. These inventions caught the public's fancy and are often mentioned in discussions of very large numbers. In the traditional American system for naming large numbers, the googol is equal to 10 duotrigintillion. **gpf**- symbol for U.S. gallons per flush, a specification sometimes found on toilets. 1 gpf = 3.785 liters per flush (Lpf). U.S. government regulations now require the use of low-flush toilets of 6.0 Lpf = 1.585 gpf or less.
**gpg**- a customary symbol for grains per gallon (gr/gal) (see below).
**gpm, gps**- customary symbols for gallons per minute (gal/min) and gallons per second (gal/s), traditional units for measuring the flow of liquids. 1 gpm equals about 3.785 41 liters per minute (L/min) if U.S. gallons are meant, or exactly 4.546 09 L/min if British Imperial gallons are meant.
**GPU**- symbol for the gas permeation unit (see above).
**gr**- an ambiguous symbol, traditionally used in English both for the grain and the gross. It has also been used, very improperly, for the gram (g). In the metric world, gr is often the symbol for the grad or grade (next entry).
**grad**,**grade [1],**or**gradian**,**gon****(**^{g}**or****gr or grd)**- a unit of angle measurement equal to 1/400 circle, 0.01 right angle, 0.9°,
or 54'. This unit was introduced in France, where it is called the
**grade**, in the early years of the metric system. The**grad**is the English version, apparently introduced by engineers around 1900. The name**gon**is used for this unit in German, Swedish, and other northern European languages in which the word*grad*means*degree*. Although many calculators will display angle measurements in grads as well as degrees or radians, it is difficult to find actual applications of the grad today. **grade [2]**- a measure of the steepness of a slope, such as the slope of a road or a
ramp. Usually stated as a percentage in the U.S, the grade is the same quantity
known as the slope in mathematics: the amount of (vertical) change in elevation
per unit distance horizontally ("rise over run"). Thus a 5% grade has an elevation
gain of 0.05 meter for each meter of horizontal distance, or 0.05 foot for
each foot of horizontal distance. Grades are also stated as ratios (1/20 grade
or 1 in 20 grade) and in some countries as a permillage (50
^{o}/oo grade). The angle of inclination, in grads or grades [1], is*not*equal to the percentage grade in this sense: for a 5% grade the angle of inclination is about 2.86° or 3.18 grads. **grade [3]**- a measure of quality for ball bearings. A bearing of grade
*g*is required to be spherical to an accuracy of*g*parts per million (*g*10^{-6}). Thus lower grade numbers represent better bearings; a 25 grade bearing is spherical to within 25 ppm, but a 1000 grade bearing is only spherical to within 1000 ppm, or 0.1%. **grade point (gp or GP)**- a unit of academic recognition used in U.S. schools and colleges. To compute the number of grade points awarded to a student for a course, the student's grade (often assigned as a letter or a percentage) is converted to a standard scale (traditionally 0.0-4.0, with 4.0 being the highest score). This scale number is multiplied by the number of semester hours or quarter hours assigned to the course. Thus a student who has a grade of A (4.0) on a course carrying 3 semester hours of credit receives 12 grade points.
**grade point average (GPA)**- an index of academic achievement used in U.S. schools and colleges, equal to the number of grade points received divided by the number of semester hours or quarter hours of courses taken by the student.
**gradian**- another name for the grad (see above).
**grain (gr) [1]**- a traditional unit of weight. The grain, equal to 1/480 troy ounce (see also pound [2]), or exactly 64.798 91 milligrams, was the legal foundation of traditional English weight systems, with various pounds being defined as a specified number of grains: 5760 grains in a troy pound and 7000 grains in an avoirdupois pound, for example. In the version of the troy system used by jewelers, there are 24 grains in a pennyweight and 20 pennyweight in an ounce. In the version used by apothecaries, there are 20 grains in a scruple, 3 scruples in a dram, and 8 drams in an ounce. Originally the grain was defined in England as the weight of a barleycorn. This made the English grain larger than the corresponding grain units of France and other nations of the Continent, because those units were based on the weight of the smaller wheat grain.
**grain (gr) [2]**- a unit of weight formerly used by jewelers in measuring diamonds and other
precious stones. The jeweler's grain is exactly 1/4 carat.
Now that the carat has been standardized at 200 milligrams, the jeweler's
grain is exactly 50 milligrams, or approximately 0.7716 troy grain. This unit
is widely used for measuring pearls, so it is sometimes called the
**pearl grain**. **grain (gr) [3]**- a traditional French unit of weight equal to 53.115 milligrams.
**grain per gallon (gr/gal or gpg)**- a traditional unit measuring the hardness of water. Water is "hard" if
it contains dissolved minerals such as calcium or magnesium salts. 1 gpg
is equivalent to about 17.118 milligrams per liter (mg/L). This unit is also
called the
**Clark degree**; see degree [4]. **gram (g)**- a unit of mass in the metric system. The name comes from the Greek
*gramma*, a small weight identified in later Roman and Byzantine times with the Latin*scripulum*or scruple (the English scruple is equal to about 1.3 grams). The gram was originally defined to be the mass of one cubic centimeter of pure water, but to provide precise standards it was necessary to construct physical objects of specified mass. One gram is now defined to be 1/1000 of the mass of the standard kilogram, a platinum-iridium bar carefully guarded by the International Bureau of Weights and Measures in Paris for more than a century. (The kilogram, rather than the gram, is considered the base unit of mass in the SI.) The gram is a small mass, equal to about 15.432 grains or 0.035 273 966 ounce. The original French spelling**gramme**is sometimes used.**Note:**The only correct symbol for the gram is g. The abbreviations "gm" and (worse) "gr" should never be used. **gram atom, gram atomic weight**- older names for the atomic mass unit.
**gram calorie**- the CGS unit of heat energy; see calorie.
**gram equivalent (gEq), gram equivalent mass (GEM), gram equivalent weight (GEW)**- various names for the mass in grams of a substance that would react with or replace one gram of hydrogen. See equivalent, the shorter name now used in many contexts for this unit.
**gram force**- see gram weight, below.
**gram mole, gram molecule, gram molecular weight (gmol or gmole)**- older names for the mole.
**gram per square meter (g/m**^{2}or gsm)- a common metric unit of areal density, used for paper, fabric, and similar
materials. One gram per square meter is equal to about 0.029 4935 ounce per
square yard. The symbol
**gsm**is commonly used for this unit, but**g/m**is the proper SI symbol.^{2} **gram rad**- a name sometimes used for the rad, a unit of radiation dose.
**gram weight or gram force (gf)**- a unit of weight equal to the force exerted on a mass of one gram by gravity
at the Earth's surface: approximately 980.665 dynes,
9.806 65 millinewtons, or about 0.002 2045
pounds of force in the English system. This unit has also been called
the
**pond**. **gran**- a traditional German weight unit, varying in size but typically about 60 milligrams.
**grand (G)**- slang for 1000, especially the sum of 1000 dollars in the U.S. or 1000 pounds in Britain.
**grano**- a traditional Italian weight unit, varying in size but typically about 50
milligrams. The plural is
**grani**. **grav (***g*)- another name for the unit of acceleration usually called the
*g*(see above). **gray (Gy)**- the SI unit of radiation dose. Radiation carries energy, and when it is absorbed by matter the matter receives this energy. The dose is the amount of energy deposited per unit of mass. One gray is defined to be the dose of one joule of energy absorbed per kilogram of matter, or 100 rad. The unit is named for the British physician L. Harold Gray (1905-1965), an authority on the use of radiation in the treatment of cancer.
**great gross**- a traditional unit of quantity equal to a dozen gross, or 1728.
**great hundred**- a traditional unit of quantity equal to 120 instead of 100. The great hundred equals 6 score or 2 shocks. Merchants in medieval England used both the great hundred and the gross (144) in specifying quantities, but more recently the gross has been much more common.
**great year**- the Platonic year.
**green ton, green tonne (GT)**- terms used in the forest products industry for a U.S. ton or metric ton (tonne) of freshly cut timber, bark mulch, etc. Once the material has dried, it will of course weigh less.
**Gregorian year**- a unit of time equal to exactly 365.2425 days, the average length of a year in the Gregorian calendar currently used throughout the world. See year [2].
**grit**- a measure of fineness for abrasive materials such as sandpaper, sanding belts, or the finer materials used to polish optical surfaces. Originally the grit number was the number of holes in a standard screen; if the screen had, say, 240 holes, then the particles that would pass through the screen were described as 240 grit. However, very fine abrasive particles (such as 500 to 1000 grit) are too small to be screened in this way, so the measure is defined by tables giving the average particle size in micrometers for each grit size. Currently there are two grit size scales in use, the CAMI scale originally created by the Coated Abrasive Manufacturers Institute and the FEPA scale created by the Federation of European Producers of Abrasives and incorporated in the ISO 6344 standard. Wikipedia has a table comparing the two scales.
**gros**- a traditional French weight unit equal to 3 deniers (about 3.824 grams).
**gross (gro or gr)**- a unit of quantity equal to a dozen dozen, or 144. This commercial unit has been in use since at least the 1400's.
**gross ton (GT)**- the name "gross ton" is used in at least two ways: (1) as another name for the British Imperial or long ton of 2240 pounds (see ton [1]), and (2) as another name for the register ton, a unit of volume equal to 100 cubic feet (see ton [3]), in describing the entire interior volume of a ship as opposed to the cargo-carrying capacity. To avoid confusion, it is better to use "long ton" for use (1) and "gross register ton" for use (2).
**ground**- an informal unit of land area in India, especially southern India, equal to roughly 200-220 square meters or 2150-2400 square feet.
**growler**- a container of beer designed for carryout. In the U.S., a growler generally holds 1/2 gallon (about 1.89 liters).
**gry**- a proposed unit of distance in the English traditional system. The name was first used in June 1679 by the philosopher John Locke (1632-1704) as a unit equal to 0.001 foot, 0.01 inch, or 0.1 line in a decimalized distance system. (Thomas Jefferson, who was very familiar with Locke's writings, later proposed a similar system in the U.S., but he called 0.001 foot a point rather than a gry.) In 1813, the gry was revived in another decimal measurement scheme in Britain. All these ideas failed, but the gry had some limited use in the nineteenth century as a unit equal to 0.1 line or 1/120 inch (0.211 667 millimeter). Long forgotten, the gry recently came back into the limelight in connection with a trick question, circulating on the Internet, which asked for three common English words ending in -gry. The word "gry" is from the ancient Greek, where it meant "a trifling amount."
**gsm**- a common but non-standard symbol for grams per square meter (g/m
^{2}), the metric unit of density for paper and for fabric. Paper density measured in this unit is often called**grammage**. **gt, gtt**- traditional pharmicist's abbreviations for a drop.
Originally, gt was the singular (1 gt) and gtt the plural. The symbol comes
from the Latin word
*gutta*for a drop. **g/t**- symbol for grams per tonne (metric ton), a unit of proportion equal to 0.001 g/kg or 1 part per million by mass. This unit is used, for example, in stating the concentration of gold, silver, or other minerals in ores.
**Gunter's chain**- the traditional surveyor's chain equal to 4 rods [1]; see chain.
**Gurley unit**- see porosity.
**gutenberg**- a unit of distance used in typography, equal to 1/7200 inch or 3.5278 micrometers. The gutenberg is 0.01 point [2], more or less, depending on how the point is defined. The unit is named for Johannes Gutenberg (ca. 1390-1468), the German inventor of printing from movable type.

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All material in this folder is copyright © 2001 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 13, 2001