I

i: a mathematical unit equal to the square root of -1. Although often called the imaginary unit, i is quite real in many applications. For example, in vector geometry it is used to represent a counterclockwise rotation by 90°. The Swiss mathematician Leonhard Euler (1707-1783) introduced the symbol i for the imaginary unit in 1777.
icfm: abbreviation for inlet cubic feet per minute, a unit traditionally used to measure the capacity of air compressors.
immi: a traditional Swiss unit of liquid volume. The immi has been brought into the metric system and now equals exactly 1.5 liters (about 3.17 U.S. liquid quarts).
imperial: a large wine bottle holding about 6 liters, 8 times the volume of a regular bottle. Also called a methuselah.
imperial units: the units of the British imperial system, adopted by Parliament in 1828. The basic units of the system are the foot, the avoirdupois pound, and the imperial pint.
inch (in or ") [1]: a traditional unit of distance, equal to 1/12 foot or 1/36 yard. One inch is now defined to be exactly 2.54 centimeters. The Old English word ynce is derived from the Latin uncia, meaning a 1/12 part; thus "inch" and "ounce" actually have the same root. The unit originated as the width of a thumb, and in many European languages the word for inch also means thumb: examples include the Dutch duim, Swedish tum, French pouce, and Spanish pulgada. See Length Converter
inch of mercury (in Hg): a traditional unit of atmospheric pressure. In the United States, atmospheric pressure is customarily expressed as the height of a column of mercury exerting the same pressure as the atmosphere. When a traditional mercury barometer is used, this height is read directly as the height of the mercury column. In the symbol for the unit, Hg is the chemical symbol for mercury. The pressure of one inch of mercury is about 0.491 pounds per square inch, or 33.87 millibars, or 3.387 kilopascals. See Converter.
inch of water column (in WC): a traditional unit of pressure, used in plumbing to describe both water and gas pressures. A pressure of 1 inch of water equals 249.09 pascals, which is 2.4909 millibars, about 0.0360 pounds per square inch (psi) or about 0.07348 inches of mercury. See Converter.
inch of water gauge (in wg or "wg): another common name for the inch of water column. The word "gauge" (or "gage") after a pressure reading indicates that the pressure stated is actually the difference between the absolute, or total, pressure and the air pressure at the time of the reading.
inch pound (in lb): a traditional unit of work or energy, equal to 1/12 foot pound, about 0.112 985 joule or 1.0709 x 10^-4Btu.
in d.: abbreviation for the Latin in die, daily, a unit of frequency used in medical prescriptions.
inhour (ih or inhr): a unit used in nuclear engineering to describe the "reactivity" of a nuclear reactor. In a reactor, fast-moving neutrons break apart atoms of uranium or plutonium; the fission of these atoms releases additional neutrons which keep the reaction going. The ratio R between the number of neutrons created and the number consumed in each cycle of fission must be very close to 1 in order for the reaction to be controlled. The reactivity is the difference k = R - 1 between this ratio and 1. One inhour is the reactivity which will cause the number of neutrons to increase by a factor of e = 2.71828 in one hour; a reactivity of t inhours will cause the number of neutrons to increase by a factor of e in 1/t hours. The exact size of the unit varies according to the design of the reactor. Enrico Fermi (1901-1954), the Italian-American physicist who built the first nuclear reactor, introduced this unit in 1947; its name is an acronym for "inverse hours." Other reactivity measures include the dollar and the milli-k.
international foot: the current foot unit of the English-speaking countries, equal to exactly 30.48 centimeters. See survey foot for additional information.
international nautical mile: the nautical mile as currently defined by international agreement, equal to exactly 1852 meters or 6076.11549 feet. This long name is sometimes used to distinguish the current nautical mile from older units.
international unit (IU): a unit used to measure the mass of certain vitamins and drugs based on their expected effects. For each substance to which this unit applies, there is an international agreement specifying the biological effect expected with a dose of 1 IU. Other quantities of the substance are then expressed as multiples of this standard. Examples: 1 IU represents 45.5 micrograms of a standard preparation of insulin or 0.6 microgram of a standard preparation of penicillin. Consumers most often see IU's on the labels of vitamin packages: the equivalent of 1 IU is 0.3 microgram (0.0003 mg) for vitamin A, 50 micrograms (0.05 mg) for vitamin C, 25 nanograms (0.000025 mg) for vitamin D, and 2/3 milligram for vitamin E.
Internet time: a global time system proposed by the Swatch Corporation. See beat for more information.
ipy: an abbreviation for inches per year (in/yr), a traditional unit for corrosion rates.
Irish acre: a traditional unit of land area in Ireland, equal to 160 square Irish perches (see next entry). This is equivalent in English units to 7840 square yards, 70 560 square feet, or about 1.6198 English acre (0.6555 hectare). The colpa, a traditional Irish unit of pasturage, is approximately equal to the Irish acre.
Irish mile: the traditional mile in Ireland is 6720 feet, which is 1.272727 English mile or 2.048 256 kilometers. The discrepancy arose because the Irish perch, or rod, was standardized at 21 feet instead of the English figure of 16.5 feet. Just as in England, the Irish chain was equal to 4 perches (84 feet instead of 66 feet), the Irish furlong was equal to 10 chains (840 feet instead of 660 feet) and the mile was equal to 8 furlongs.
ISO: the "short name" of the International Organization for Standardization. Based in Switzerland, the ISO sets international industrial standards, including standards for the use of units of measurements.