Planck's constant, equal to exactly 6.626 070 15 x 10-34 joule second, a fundamental constant of physics also used as a unit of "action" or of angular momentum in particle physics. The unit was defined by the German physicist Max Planck (1858-1947), who showed in 1900 that at atomic and subatomic scales energy occurs in discrete packets called quanta. Each quantum has energy h·f, where f is the frequency of the radiation in hertz (see below). In November 2018 CGPM adopted an exact value for h as its definition of the kilogram.
a large traditional unit of land area in Mexico and the southwestern U.S. The word also refers to a large estate, ranch, or plantation. As a unit, it equals 5 square leguas or 125 million square varas. Using the Texas definition of the vara, this would be about 8960 hectares or 22 140 acres (34.59 square miles). Using the shorter Mexican vara, it would be about 8778 hectares or 21 690 acres.
hair's breadth
a common informal unit of distance. Human hairs vary considerably in width, depending on age, color, genetics, and other factors. An average hair is approximately 70 to 100 micrometers (µm) in diameter, so, as a rough standard, a hair's breadth is 100 µm or 0.1 millimeter (mm).
halakim (hl)
plural of helek (see below). The word is also transliterated as chalakim or chlakim.
a German word for one half; often used as a unit of volume for beer and other liquids equal to 1/2 maß (generally 1/2 liter).
half [1]
a unit of proportion equal to 1/2. The English word "half," like the German prefix "halb-," is often placed before the name of a unit to create a combination which functions as a new unit equal to half the old one. Half dozen, half hour, and half gallon are typical and common examples.
half [2]
an informal name for 1/2 of many units. For example, in Britain a "half" often means a 1/2 pint glass of beer, cider, lemonade, or whatever.
half life
a unit of relative time measuring the rate at which a radioactive substance decays, or (more generally) the rate of decrease for any process that decreases exponentially. In the case of radioactivity, the half life is the time required for the activity to be reduced by half. After a second half life, the activity is again reduced by half, so it is then 1/4 the original activity. To reduce the activity to 0.1% of the original amount requires about 9.966 half lives.
a unit of time used in a formal way in astronomy to construct official designations for comets and provisional designations for newly discovered minor planets. The 24 half-months in the year are designated with letters A through Y (the letter I is not used). As an example, the famous Comet Hale-Bopp has the official designation C/1995 O1, indicating that it was the first comet discovered during the second half of July (letter O) in 1995. The minor planet "Utima Thule" visited by the New Horizons spacecraft in 2019 had the provisional designation 2014 MU69; 2014M represents the second half of June 2014 when the minor planet was discovered. (U69 is a code indicating that Ultima Thule was the 1745th minor planet reported during that half month: 1745=25·69+ 20 for the letter U. Ultima Thule has since received a permanent designation as minor planet 486958.)
half step
a unit used in music to describe the ratio in frequency between notes. Equal to 1/12 octave, the half step measures the difference between adjacent notes in the standard 12-tone scale, as on a piano keyboard. Two notes differ in frequency by a half step if the higher one has frequency equal to 21/12 = 1.0595 times the frequency of the lower one.
a traditional unit of distance, now used mostly to measure the height of horses. One hand equals 4 inches, 1/3 foot, or 10.16 centimeters.
a traditional unit of volume for beer, used in pubs in the Northern Territory of Australia. A handle of beer is 285 milliliters (10 fluid ounces). Glasses of this size are called middies or pots in most Australian states, schooners in South Australia.
a traditional measure of length for yarn. The length of yarn in a hank varies with the market and the material; for example, a hank of cotton yarn traditionally included 840 yards (768 meters) of yarn, while a hank of wool yarn was 560 yards (512 meters). For both cotton and wool, these traditional hanks are equal to 7 leas or to 12 cuts. In the U.S., however, a hank of woolen yarn is generally 1600 yards (1463 meters). In retail trade, a hank is often equal to 6 or 7 skeins of varying size.
a measure of the hardness of a metal or mineral. Hardness is a property easy to appreciate but difficult to quantify and measure. The Mohs hardness scale is used in geology to give a rough estimate of hardness by testing which minerals are able to scratch the sample. In metallurgy, samples are tested for hardness by machines which indent the surface under a controlled pressure; the resulting measurement is often computed as the force applied divided by the surface area of the indentation. The Brinell, Vickers, Rockwell, and Knoop tests are among the techniques used. Plastics, rubber, and similar materials are tested with instruments called durometers and the resulting readings are often designated duro.
hartley (Hart)
a unit of information content used in information and communications theory. The hartley is similar to the shannon. If the probability of receiving a particular message is p, then the information content of the message is -log10 p hartleys. For example, if a message is a string of 5 letters or numerals, with all combinations being equally likely, then a particular message has probability 1/365 and the information content of a message is 5(log10 36) = 7.7815 hartleys. One hartley equals log2 10 = 3.321 928 shannons or loge 10 = 2.302 585 nats.
hartree (Ha or Eh)
a unit of energy used in atomic and molecular physics, equal to 2 rydbergs or about 4.3597 x 10-18 joule or 27.213 electron volts. The unit is named for the British physicist and mathematician Douglas R. Hartree (1897-1958).
hat size
in the metric world, the size of a hat is simply the circumference C of the head it's meant to cover, in centimeters. The traditional U.S. hat size was defined to equal C, in inches; this would be the diameter if the head were circular in cross section. Thus the U.S. hat size equals the metric size divided by 2.54π, which is almost exactly 8. Traditional British hat sizes are equal to U.S. sizes minus 1/8. Thus metric size 60 is equivalent to U.S. size 60/8 = 7 1/2 and British size 7 3/8.
hat trick or hat-trick
an informal unit of quantity equal to 3. The unit is used in sports to describe three successes by a player either consecutively or in a single contest. It originated in cricket to describe the very rare event of a bowler dismissing three batsmen on consecutive throws. No one seems to know the reason for sure: according to one popular account, cricket clubs in England sometimes awarded a hat to a player who performed the feat, but others suspect this was an effect, rather than a cause, of the expression. Today "hat trick" is more familiar to North Americans in ice hockey, where it refers to the less rare event of a player scoring three goals in a single game.
head (hd) [1]
an informal unit of length, equal to the approximate length of a horse's head, used in expressing the results of a horse race.
head (hd) [2]
a notation seen in measurements of water pressure; see foot of head.
head (hd) [3]
a unit of quantity for livestock, equal to one animal. No -s is added for the plural: one speaks of "35 head" of cattle.
heaped bushel
a traditional unit of volume in the United States, the heaped bushel is just what its name implies: the volume of a bushel container filled to overflowing. Officially, the heaped bushel was supposed to equal 1.278 regular, or "struck" bushels; this is a volume of 2748.237 cubic inches, 1.5904 cubic feet, or 45.036 liters. In practice, the heaped bushel was frequently interpreted as 1.25 bushels, which is equal to exactly 5 pecks, 2688.025 cubic inches, 1.5556 cubic feet, or 44.049 liters.
heat index (HI or HX)
a measure of the combined effect of heat and humidity on the human body. U.S. meteorologists compute the index from the temperature T (in °F) and the relative humidity H (as a fraction; that is, H = 0.65 if the relative humidity is 65%). The formula used is
HI = -42.379 + 2.04901523 T + 1014.333127 H - 22.475541 TH - .00683783 T2 - 548.1717 H2 + 0.122874 T2H + 8.5282 TH2 - 0.0199 T2H2.
heating degree day (HDD)
see degree day.
heat unit
an alternate name, coined by Americans, for the British thermal unit (Btu). This is not the same as the Celsius heat unit (Chu).
an obsolete metric prefix denoting ten million (107). It was coined from the Greek hebdomos, seventh. A group of seven is sometimes called a hebdomad, a word related to the French hebdomadaire, weekly.
hect- or hecto- (h-)
a metric prefix denoting 100, coined from the Greek word hekaton for one hundred.
a unit of quantity equal to 100. In the Ordnance Survey mapping of the British Isles a hectad is a square 10 kilometers on a side (thus covering 100 square kilometers).
hectare (ha)
the customary metric unit of land area, equal to 100 ares. One hectare is a square hectometer, that is, the area of a square 100 meters on each side: exactly 10 000 square meters or approximately 107 639.1 square feet, 11 959.9 square yards, or 2.471 054 acres.
hectare meter (ha·m)
a unit of volume used to measure the capacity of reservoirs, equal to the volume of water one meter deep covering one hectare. The unit is used mostly in British Commonwealth countries, especially India, where it provides a metric unit comparable to the traditional English acre foot. Reservoir capacities are often stated in millions of hectare meters (Mha·m or MHM). One hectare meter equals exactly 10 000 cubic meters or about 8.1071 acre feet.
hectobar (hbar)
a fairly common metric unit of pressure equal to 100 bars, 10 megapascals (MPa), or 1 dekanewton per square millimeter (daN/mm2). This is approximately 1450.38 pounds per square inch (lbf/in2 or psi) or 208 855 pounds (104.43 U.S. tons) per square foot.
hectofoot (hft)
a unit of distance equal to 100 feet. The only known use for this odd unit is in statements of the assigned flight level of an aircraft.
hectogram (hg)
a common metric unit of mass, equal to 100 grams or about 3.5274 ounces.
hectokilogram (hkg)
a non-standard metric unit of mass equal to 100 kilograms or about 220.462 26 pounds. This unit was fairly common in the past, especially in agriculture, but since the SI now forbids applying more than one prefix to a metric unit the hectokilogram is now considered to be obsolete.
hectoliter (hL or hl)
a common metric unit of volume. The hectoliter equals 100 liters, 0.1 cubic meter, 26.417 U.S. liquid gallons, 21.999 British Imperial gallons, or 3.5315 cubic feet.
hectometer (hm)
a metric unit of distance equal to 100 meters, 328.084 feet, or 109.361 yards. This unit isn't used much in everyday life in metric countries, but it appears in various scientific contexts.
hectopascal (hPa)
a metric unit of pressure equal to 100 pascals or 0.1 kilopascal (kPa). The hectopascal, used almost entirely in measurements of air pressure, is identical to the millibar (mb). The millibar has been used to measure air pressure for many years, but it is not an SI unit. Although meteorologists in various countries use the hectopascal as a kind of alias for the millibar, the natural SI unit for air pressure is the kilopascal.
a traditional measure of length for linen and woolen yarn, equal to 2 cuts or 1/6 hank (see above). This is equivalent to 80 yards (73.152 meters).
Hefner candle (HC) or Hefnerkerze (HK)
a former unit for measuring the intensity of light. The unit is named for F. von Hefner-Altenack (1845-1904), who invented a laboratory light standard, the Hefner lamp, which burned isopentyl acetate to provide light of intensity 1 HK. One Hefner candle is equivalent to approximately 0.902 candela (cd).
Hegman unit
a unit measuring how finely ground the particles of pigment (or other solid) dispersed in a sample of paint (or other liquid) are. The unit is equal to 0.5 mil, or 12.7 microns, but the scale is inverted: 0 mil is defined to be 8 Hegman units and 4 mils equal 0 Hegman units. In other words, if the particles are of average diameter x mils, the fineness is 8 - 2x Hegman units.
helek (hl)
a traditional Hebrew unit of time equal to 1/1080 hour, 1/18 minute or 10/3 seconds. The plural is halakim. Halakim are used in formulas establishing the instant of new moon, which marks the start of the month in the Jewish calendar. In English, the unit is often called a part of an hour. The Jews inherited this unit from the Babylonians; it was widely used throughout the ancient Middle East.
the shortest named unit of relative time in music, equal to 1/8 quaver, 1/64 whole note or 1/128 breve.
a Roman unit of liquid volume, equal to 1/2 sextarius or about 265.6 milliliters (0.561 U.S. pint or 0.467 British Imperial pint).
a traditional unit of solid angle equal to 1/2 sphere, 2pi steradians, or about 20 626.48 square degrees.
henry (H)
the SI unit of electric inductance. A changing magnetic field induces an electric current in a loop of wire (or in a coil of many loops) located in the field. Although the induced voltage depends only on the rate at which the magnetic flux changes, measured in webers per second, the amount of the current depends also on the physical properties of the coil. A coil with an inductance of one henry requires a flux of one weber for each ampere of induced current. If, on the other hand, it is the current which changes, then the induced field will generate a potential difference within the coil: if the inductance is one henry a current change of one ampere per second generates a potential difference of one volt. The henry is a large unit; inductances in practical circuits are measured in millihenrys (mH) or microhenrys (µH). The unit is named for the American physicist Joseph Henry (1797-1878), one of several scientists who discovered independently how magnetic fields can be used to generate alternating currents. The plural is sometimes spelled henrys, but in English it is correct to spell it henries.
a unit of quantity equal to 7.
hertz (Hz)
the SI unit of frequency, equal to one cycle per second. The hertz is used to measure the rates of events that happen periodically in a fixed and definite cycle; the becquerel, also equal to one "event" per second, is used to measure the rates of things which happen randomly or unpredictably. Multiples of the hertz are common: the frequencies of radio and television waves are measured in kilohertz (kHz), megahertz (MHz), or even gigahertz (GHz), and the frequencies of light waves in terahertz (THz). The unit is named for the German physicist Heinrich Rudolf Hertz (1857-1894), who proved in 1887 that energy is transmitted through a vacuum by electromagnetic waves.
a unit of quantity equal to 6.
a unit of information equal to 4 bits or 1/2 byte. A string of 4 bits has 16 possible states (0-15) and is usually represented as a single base-16, or hexadecimal, digit (in the hexadecimal system the letters A through F are used to represent the numbers 10 through 15, respectively). A hexit of data is also known as a nibble or a quadbit.
a very old English unit of land area, dating from perhaps the seventh century. The hide was the amount of land that could be cultivated by a single plowman and thus the amount of land necessary to support a family. Depending on local conditions, this could be as little as 60 acres or as much as 180 acres (24-72 hectares). The hide was more or less standardized as 120 acres (48.6 hectares) after the Norman conquest of 1066. The hide continued in use throughout medieval times, but it is now obsolete. The unit was also known as the carucate.
an ancient Hebrew unit of liquid capacity, mentioned several times in the Bible. The unit varied in size over time, but typically it was around 3.7 liters, a little smaller than the U.S. gallon.
hogshead (hhd)
a traditional unit of volume for liquids. Originally the hogshead varied with the contents, often being equal to 48 gallons of ale; 54 of beer; 60 of cider; 63 of oil, honey, or wine; or 100 of molasses. In the United States, a hogshead is defined to hold 2 barrels, or 63 gallons; this was the traditional British wine hogshead. It is equal to exactly 14 553 cubic inches, or about 8.422 cubic feet (238.48 liters). In the British Imperial system, the hogshead equals 1/2 butt, or 52.5 Imperial gallons (8.429 cubic feet, or 238.67 liters). Thus the British Imperial and American hogsheads are almost exactly the same size. No one seems to know for sure how this unit got its unusual name.
one of two Hungarian units of land area. The traditional magyar hold or Hungarian acre is equal to 1200 square öl (fathoms) or about 0.4314 hectare (1.066 acres). The official kataszteri hold or cadastral hold, used for land taxation, is 1600 square öl or about 0.5752 hectare (1.421 acres); this unit is equivalent to the Austrian joch.
a historic unit of area in the United States, equal to 160 acres (64.75 hectares). Under the Homestead Act passed by Congress in 1862, settlers in the western states were allowed to take title to a homestead of 160 acres of land by registering a claim, settling on the land, and cultivating it. A homestead equals 1/4 square mile, or 1/4 section in U.S. government terminology.
hoppus foot, hoppus board foot
traditional units of volume in British forestry. In a 1736 manual of practical calculation, the English surveyor Edward Hoppus advised foresters to estimate the volume of wood in a log of length L and girth (circumference) G as L·(G/4)2. Since the actual volume of a cylinder is L·G2/(4·π), the resulting figure, called the hoppus volume, is smaller than the actual volume of the log. In fact not all the wood in a log can be used, and the hoppus volume was considered a fairly reasonable estimate of the usable volume of wood in the log. Volume measurements made using the hoppus formula are stated in hoppus feet. In effect, this makes the hoppus foot a unit of volume equal to 4/π = 1.273 cubic feet or 0.036 054 cubic meter. Similarly, the hoppus board foot is equal to 1/12 hoppus foot or 1.273 board feet, which is almost exactly 3 liters (0.00300 cubic meter). The British forestry industry changed its unit of timber measurement from hoppus feet to cubic meters in 1971.
hoppus ton (HT)
a traditional unit of volume in British forestry. One hoppus ton is equal to 50 hoppus feet or 1.8027 cubic meters. Shipments of tropical hardwoods from Southeast Asia, especially shipments of teak from Myanmar (Burma), are still stated in hoppus tons.
slang for the horsepower, as in "200-horse engine."
horsepower (hp)
a unit of power representing the power exerted by a horse in pulling. The horsepower was defined by James Watt (1736-1819), the inventor of the steam engine, who determined after careful measurements that a horse is typically capable of a power rate of 550 foot-pounds per second. This means that a horse, harnessed to an appropriate machine, can lift 550 pounds at the rate of 1 foot per second. Today the SI unit of power is named for Watt, and one horsepower is equal to approximately 745.6999 watts. (Slightly different values have been used in certain industries.) Outside the U.S., the English word "horsepower" is often used to mean the metric horsepower, a slightly smaller unit.
horsepower hour (hp hr)
a unit of work or energy equal to the work done at the rate of 1 horsepower for 1 hour. The horsepower hour equals 1 980 000 foot pounds or approximately 2.685 megajoules, 2545 Btu, 641.1 (large) Calories, or 745.7 watt hours.
horsepower year (hp yr)
an obsolete unit of work or energy equal to the work done at the rate of 1 horsepower for 1 year. Based on the Julian year of 365.25 days, the horsepower year is approximately 6536.8 kilowatt hours, 23.537 gigajoules, or 22.309 million Btu.
Hounsfield unit (HU)
a unit used in medical imaging (CT or MRI scanning) to describe the amount of x-ray attenuation of each "voxel" (volume element) in the three-dimensional image. The voxels are normally represented as 12-bit binary numbers, and therefore have 212 = 4096 possible values. These values are arranged on a scale from -1024 HU to +3071 HU, calibrated so that -1024 HU is the attentuation produced by air and 0 HU is the attenuation produced by water. Tissue and bone then produce attenuations in the positive range. The reading in Hounsfield units is also called the CT number. The unit is named for the British engineer Godfrey Hounsfield, who demonstrated the first CT scanner in 1972. For this invention he received the Nobel Prize in medicine in 1979.
hour (h or hr) [1]
a traditional unit of time, equal to 60 minutes, or 3600 seconds, or 1/24 day [2]. The custom of dividing the daylight into 12 hours goes back at least as far as the Babylonians, who liked to divide units by 12 because groups of 12 are easily divided into halves, thirds, or fourths. Originally an hour was 1/12 of the time between sunrise and sunset, so summer hours were longer than winter hours. Later, when people wanted to express times at night, it was natural to divide the night into 12 hours as well, making 24 hours in the day. Only after the invention of mechanical clocks, around 1300, did hours became equal intervals marked by clocks. The word comes from an ancient Greek word hora which originally meant a season, especially a religious season, and hence a "defined" period of time. In the Christian church hora came to mean one of the services held at seven specific times during the day, thus establishing the word as marking subdivisions of the day.
hour (h or hr) [2]
a unit of angular measure used in astronomy, equal to 1/24 circle or 15°. Objects can be located in the sky in a coordinate system in which the equatorial plane is the same as that of the Earth. In this system, the latitude coordinate is called declination and is measured in degrees from the Equator to the poles, just as latitude is measured on the surface of the Earth. The longitude coordinate, called right ascension, is measured in hours from the longitude, traditionally known as the First Point of Aries, at which the Sun appears to cross the Equator on its northward journey in the spring.
hour (h or hr) [3]
a unit of sidereal time in astronomy; see sidereal day.
in astrology each sign of the Zodiac is called a house; thus the house can be considered a unit of angle measure equal to 1/12 circle or 30°.
a basic unit of analysis in economics and sociology equal to the average number of persons who live and eat together as a social unit. Obviously this varies geographically and over time. In recent years the average household in the U.S. has been at or close to 2.54 persons.
a traditional Chinese unit of liquid volume. The hu contains about 51.77 liters, 13.676 U.S. gallons, or 11.389 British Imperial gallons.
a unit of distance sometimes used in astronomy. The hubble is a gigantic unit, equal to 109 light years. This is 9.4605 x 1021 kilometers (9.4605 yottameters, if you please), 5.8785 sextillion miles (U.S.), 63.240 x 1012 astronomical units, or 306.595 megaparsecs. In practice, most astronomers use the megaparsec for measuring such stupendous distances. The unit honors the American astronomer Edwin Hubble (1889-1953), who discovered the expansion of the Universe later explained by the Big Bang theory.
a numerical measure of color; see color units.
a unit of length in Thailand, equal to 1/8 inch or 3.175 millimeters. Pronounced "hoon," the unit seems to be used almost entirely for measuring the size of dice.
hundred [1]
a unit of quantity. In commercial use in old England the term "hundred", or cent (C), did not always mean an even 100; sometimes it meant 120 (the "great hundred") or some other number, depending on the item. For fish, the exact number in a hundred varied with the species.
hundred [2]
an old English unit of area equal to 100 hides (see above). This is roughly 12 000 acres, 5000 hectares, or 18.75 square miles. The hundred is approximately the area of a village with its associated fields, so the name "hundred" came to mean a minor division of a shire or county. This use carried over to the American colonies, where, for example, many of the early settlements in Virginia were called hundreds. In colonial South Australia, the hundred was a subdivision of a county with an area of about 100 square miles or 260 square kilometers.
hundredweight (Cwt or cwt)
a traditional unit of weight equal to 1/20 ton. The hundredweight is the English version of a commercial unit used throughout Europe and known in other countries as the quintal or the zentner. In general, this unit is larger than 100 pounds avoirdupois, so to fit the European market the hundredweight was defined in England as 112 pounds avoirdupois (about 50.8023 kilograms) rather than 100 pounds. This definition apparently dates from about the middle of the 1300's. The British hundredweight was divided into 4 quarters [1] of 28 pounds, 8 stone of 14 pounds, or 16 cloves of 7 pounds each. In the United States, where the currency was decimalized and there wasn't so much need to align the unit with the quintal and zentner, the hundredweight came to equal exactly 100 pounds (about 45.3592 kilograms). The U.S. hundredweight seems to have been invented by merchants around 1840. To distinguish the two hundredweight units, the British version is often called the long hundredweight and the American is called the short hundredweight or cental. The C in the symbol is of course the Roman numeral 100.
the Hungarian inch unit, equal to 1/12 láb or about 2.63 centimeters. As is true of the inch unit in many languages, the word also means "thumb."
an obsolete MKS unit of mass. One hyl is the mass that is accelerated at one meter per second per second by one kilogram of force (kgf). Since 1 kgf = 9.806 65 newtons, the hyl is equivalent to 9.806 65 kilograms. The name of the unit comes from an ancient Greek word for matter. The hyl is known in German engineering as the Technische Mass Einheit (engineering mass unit) or TME; in English it is called the metric slug, or "mug" for short.


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July 11, 2000