H

h: Planck's constant, equal to approximately 0.662606876×10^-33joule 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).
hacienda: 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, of about 8,960 hectares or 22,140 acres would be about (34.59 square miles). Using the shorter Mexican vara, it would be about 8.778 hectares or 21,690 acres, or 122.6 million varas..
halakim (hl): plural of helek (see below).
half: 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 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.
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 2^1/12 = 1.0595 times the frequency of the lower one. See Pythagorean Comma's
hand: 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. See Length
hank: 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). In the U.S., however, a hank of wollen yarn is generally 1600 yards (1463 meters). For both cotton and wool, the hank is equal to 7 leas or to 12 cuts. In retail trade, a hank is often equal to 6 or 7 skeins of varying size.
hardness: 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 -log₁₀p 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.
hartree (E_h): a unit of energy used in atomic and moleclar physics, equal to about 4.3598 x 10^-18joule or 27.212 electron volts. The unit is named for the British physicist and mathematician Douglas R. Hartree (1897-1958).
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 10 dry gallons, 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).
hebdo-: 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. See Prefix
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: approximately 107 639.1 square feet, 11 959.9 square yards, or 2.471 054 acres. See Area
hectogram (hg): a common metric unit of mass, equal to 100 grams or about 3.5274 ounces.
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 36.4538 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 proper SI unit for air pressure is the kilopascal.
helek (hl): a traditional Hebrew unit of time equal to 1/18 minute or 10/3 seconds. The plural is halakim. Halakim are used in formulas establishing the molad, the instant of new moon, which marks the start of the month in the Jewish calendar.
hemidemisemiquaver: the shortest unit of relative time in music, equal to 1/8 quaver, 1/64 whole note or 1/128 breve.
hemina: 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).
hemisphere: a traditional unit of solid angle equal to 1/2 sphere, 2pisteradians, 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.
heptad: 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.
hexad: a unit of quantity equal to 6.
hexit: 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.
hexadecimal: a number sytem use in computer technology utilizing 16 places 0,1,2,3,4,5,6,7,8,9.a.b.c.d.e.f. See numberbase, and enter base 16.
hide: an old English unit of land area. 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 60-180 acres or (24-72 hectares). 120 acres (48.6 hectares) is a commonly used estimate.
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.
homestead: 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.
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 mega joules, 2545 Btu, 641.1 (large) Calories, or 745.7 watt hours.
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. 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. 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. 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.
hour (h or hr) [2]: a unit of angular measure used in astronomy, equal to 1/24 circle or 15°. Objects are located in the sky by latitude and "right ascension". Latitude is measured in degrees from the equator to the poles, just as it is on the surface of the earth. Longitude is replaced by right ascension, measured in hours from the longitude, traditionally known as the First Point of Aries, at which the Sun crosses the equator on its northward journey in the spring.
house: 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°.
hu: 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.
hubble: 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 10¹²astronomical units, or 306.595 megaparsecs. In practice, most astron- omers use the megaparsec for measuring such stupendous distances. The unit honors the American astronomer Edwin Hubble (1889-1953), who discovered the uniform expansion of the Universe explained by the Big Bang theory.
hue: a numerical measure of color; see color units.
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.
hundredweight (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. In the United States, where 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). To distinguish these two 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. See Mass
hyl: 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 = 9.80665 newtons, the hyl is equivalent to 9.806 65 kilograms. The name of the unit comes from an ancient Greek word for matter.