the SI symbol for the watt (see below).
a traditional unit of distance in Thailand, now aligned with the metric system as exactly 2 meters (6.562 feet). This unit is the Thai version of the fathom. It is seen mostly in connection with the talangwah or square wah (4 square meters or 4.784 square yards), a common unit of area in Thailand.
in Britain, Wales has long served as an informal unit of area, much as Rhode Island has been used in the U.S. Wales has an area of about 8015 square miles or 20 760 square kilometers; it is 7.67 times the size of Rhode Island.
a unit of quantity in China equal to 10 000. In Chinese, the wan is used much as the thousand is used in the West, as a basic unit for large quantities. Thus 100 000 is 10 wan, and 1 000 000 is 100 wan. However, as in the case of the Greek word myrios (myriad), the word wan is also used in Chinese to mean an indefinitely large number.
watch [1]
a traditional unit of time, defined as the time a sentry stands watch or a ship's crew is on duty. In ancient Rome, the night was divided into four watches, each roughly three hours long. More recently, one watch is usually equal to 4 hours on both land and sea. At sea, the evening watch (16-20 hours, or 4-8 pm) is often divided into two shorter watches called "dog watches." When dog watches are in effect, sailors will have watch assignments that rotate through the day instead of falling at the same hours every day. Watches at sea are divided into 8 bells (4 bells for dog watches). The word watch is derived from an old English word wæccan which meant "stay awake."
watch [2]
another name for a shift. This use was popularized in the U.S. by CNN Headline News and by the NBC television series Third Watch.
water column (WC)
a notation seen in pressure measurements. See inch of water, centimeter of water, or millimeter of water.
water horsepower (whp or Whp)
a unit of power used in the U.S. primarily in rating pumps. If a pump has a capacity of Q gallons per minute and develops a pressure ("head") of P feet of head, then its power rating is QP/3956 water horsepower. This calculation assumes the density of water to be 8-1/3 pounds per U.S. gallon, which is approximately correct but not exact. As a result, the water horsepower equals 746.043 watts (550.253 foot pounds per second), slightly more than the ordinary mechanical horsepower.
water inch
a traditional unit of water flow, supposed to equal the flow through a circular opening one inch in diameter, assuming the flow is caused only by gravity. However, this flow rate also depends on the pressure of the water above the opening. One estimate is 14 U.S. pints per minute or 2520 U.S. gallons per day [2] (this is equivalent to 6.624 liters per minute); this estimate assumes the water level is constantly 1/12 inch (1 line) above the top of the opening. Another is 500 cubic feet per day, which is much larger: 3740 U.S. gallons per day or about 9.832 liters per minute. The latter estimate may depend on a mid-nineteenth century British engineering definition which required the hole to be centered 1 inch and 1 line below the water surface, placing the top of the opening 7/12 inch (7 lines) below the water level. See also miner's inch.
watt (W) [1]
the SI unit of power. Power is the rate at which work is done, or (equivalently) the rate at which energy is expended. One watt is equal to a power rate of one joule of work per second of time. This unit is used both in mechanics and in electricity, so it links the mechanical and electrical units to one another. In mechanical terms, one watt equals about 0.001 341 02 horsepower (hp) or 0.737 562 foot-pound per second (lbf/s). In electrical terms, one watt is the power produced by a current of one ampere flowing through an electric potential of one volt. The name of the unit honors James Watt (1736-1819), the British engineer whose improvements to the steam engine are often credited with igniting the Industrial Revolution.
watt (W) [2]
a unit used to describe light bulbs. Incandescent light bulbs have always been sold in standard wattages such as 100 watt and 60 watt. These bulbs are now being replaced by more efficient sources such as compact fluorescent (CFL) or light-emitting diode (LED) "bulbs." For marketing purposes, these devices are often marked with the wattage of the incandescent bulbs which they are intended to replace. (CFLs require very roughly 1/4 the power of incandescent bulbs, and LEDs very roughly 1/8).
watt hour (W·h)
a common metric unit of work or energy, representing the energy delivered at a rate of one watt for a period of one hour. This is equivalent to exactly 3.6 kilojoules (kJ) of energy, or about 3.412 141 Btu, 0.859 846 (kilogram) Calories, or about 2655 foot pounds.
watt second (W·s)
a metric unit of energy, representing the energy delivered at a rate of one watt for a period of one second. Of course the watt is defined as one joule per second, so "watt second" is simply another name for the joule. However, the power delivered per pulse by devices such as flash bulbs and strobe lights is often stated in watt seconds rather than joules.
watt year (W·yr)
a metric unit of energy, representing the energy delivered at a rate of one watt for a period of one year. The watt year, equal to about 31.5569 megajoules (MJ), is used in discussions of the rate of global warming.
wave or wavelength
a unit of relative distance equal to the length of a wave: this could be a light wave, a radio wave, or even an ordinary water wave. In communications engineering, the length of an antenna is often stated in waves. In optics, the surfaces of lenses and mirrors are sometimes required to be polished to within a very small fraction of a wavelength of green light (546 nanometers).
weber (Wb)
the SI unit of magnetic flux. "Flux" is the rate (per unit of time) at which something crosses a surface perpendicular to the flow. If the something is a magnetic field, then the magnetic flux across a perpendicular surface is the product of the magnetic flux density, in teslas, and the surface area, in square meters. If a varying magnetic field passes perpendicularly through a circular loop of conducting material, the variation in the field induces a electric potential in the loop. If the flux is changing at a uniform rate of one weber per second, the induced potential is one volt. This means that numerically the flux in webers is equal to the potential, in volts, that would be created by collapsing the field uniformly to zero in one second. One weber is the flux induced in this way by a current varying at the uniform rate of one ampere per second. The weber is a large unit, equal to 108 maxwells, and practical fluxes are usually fractions of one weber. (Because of this, when we want to induce an electric potential in a conductor with a changing field, as we do in all electric generators, transformers and electric motors, we loop the conductor into hundreds of coils, thus adding together the small voltages induced in each loop by the changing field.) The unit honors the German physicist Wilhelm Eduard Weber (1804-1891), one of the early researchers of magnetism.
week (wk)
a traditional unit of time equal to seven days [3]. The custom of the seven-day week, with one day set aside for rest and religious observance, goes back more than 3000 years to the ancient civilizations of the Middle East. The seven days originally had an astrological significance; there is one day for each of the five visible planets and one each for the sun and the moon. Christians and Moslems inherited the seven-day cycle from the Jewish religion. The Romans picked up the idea from the Persians and were using the week as early as the first century. When the Emperor Constantine legalized Christianity in the Roman Empire, early in the fourth century CE, the Christian version of the week, with Sunday as the day of religious observance, became official throughout the Empire. Since none of the units of Roman date-keeping (the month, the quarter, and the year) equal a whole number of weeks, this made it necessary for the first time to have tables (we call them calendars!) showing the ever-changing relationship between the days of the week and the dates of the month. Link: The World Calendar Association promotes efforts to reform the calendar so that weeks and months would have a fixed relationship. Link: A perpetual calendar provides calendars for any month and year in the current (Gregorian) calendar.
Note: There are different traditions as to which day of the week is the first. In the U.S., most calendars show Sunday as the first day of the week, but the International Organization for Standardization (ISO) specifies that the week begins with Monday. There are also different ideas about how to number the weeks of the year, which is sometimes necessary for business purposes. The official solution to this question is that week 1 of the year is the week (beginning with Monday) that contains January 4. By this convention, week 1 of 2010 will be the week January 4-10, 2010. Link: ISO 8601 from Markus Kuhn.
a German spelling, sometimes seen in English as well, for a Russian distance unit, the verst.
wet ton, dry ton
units used to measure sludge, slurries, compost, and similar mixtures in which solid material is soaked with or suspended in water. A wet ton is an ordinary ton [1] of the material in its natural, wet state; a dry ton is a larger quantity of the slurry, containing a ton of the solid material plus a variable amount of water.
a historic English unit. The word comes from the old English wæge, meaning weight, and originally the wey was a weight unit representing about two hundredweight. Later it came to be used as a volume unit for a variety of dry commodities. Its size varied. Roughly speaking, the wey represented about 40 bushels, 2 cubic yards, or 1.5 cubic meters.
whole note
a unit of relative time in music, also called a semibreve.
whole step, whole tone
alternate names for the step [2], a unit used in music to express the ratio in frequency between two tones.
Winchester bushel
the traditional British name for what is now the U.S. bushel; see bushel [1].
Winchester quart
an informal British unit of volume used for certain chemicals shipped in cylindrical, narrow-necked bottles. A Winchester quart originally held 2 British Imperial quarts (about 2.273 liters); now it generally holds exactly 2.5 liters. This unit is not related to the Winchester bushel, and the origin of its name is not known.
wind chill temperature index (WCTI)
a measurement of the combined cooling effect of low air temperature and wind on the human body. The index was first defined by the American Antarctic explorer Paul Siple in 1939. As originally used by U.S. meteorologists, the wind chill index (WCI) was computed from the temperature T (in °F) and wind speed V (in mi/hr) using the formula:
WCI = 0.0817(3.71 sqrt(V) + 5.81 - 0.25V)(T - 91.4) + 91.4.
In 2001, U.S. and Canadian meteorologists agreed on a revised definition of the wind chill temperature index (WCTI). The U.S. version, with the temperature in °F and wind speed in miles per hour, is
WCTI = 35.74 + 0.6215T - 35.75V0.16 + 0.4275TV0.16.
The Canadian formula, with temperatures in °C and wind speed in km/h, is
WCTI = 13.12 + 0.6215T - 11.37V0.16 + 0.3965TV0.16.
The U.S. National Weather Service has a chart. The new formula gives higher temperatures, but these temperatures, unlike the old WCI numbers, are supported by clinical experiments.
wine gallon
a former English unit of volume equal to 231 cubic inches. The wine gallon was adopted as the official gallon for liquid measurement in the United States, so now it is usually called the U.S. liquid gallon.
an informal unit of volume used in U.S. bartending, equal to 4 (U.S.) fluid ounces or about 118.3 milliliters. This unit is the same as the traditional gill.
a unit of volume used in British food recipes. The wineglass holds 2.5 (British Imperial) fluid ounces, 5 tablespoonfuls, 1/2 gill, or about 71.0 milliliters. One wineglassful is equal to 0.30 U.S. cup.
word [1]
a unit of information in typing. Typing speed is usually expressed in words per minute (wpm). For this purpose, a "word" is considered to be exactly 5 characters (spaces included). For example, a typing speed of 30 wpm is equivalent to 150 characters per minute or a keystroke rate of 2.5 hertz.
word [2]
a unit of information in computer science, often representing the amount of data processed by a computer in a single instruction. The size of a word in bits is a characteristic of the computer system.
working level (WL)
a unit of radiation exposure used for measuring exposure to radon gas in the U.S. One working level represents a concentration of short-lived radon decay products in one liter of air resulting in the ultimate emission of 1.3 x 105 MeV or 20.8 nanojoules (nJ) of energy. Exposures are measured in working level months (WLM). Cumulative exposure of workers is measured in working level months (WLM), representing exposure to one working level for 170 hours. In underground mining, U.S. law says miners must not be exposed to levels exceeding 1 WL and cumulative exposure must not exceed 4 WLM per year. In other industries, exposures are limited to 0.3 WL. In homes, the U.S. Enviromental Protection Agency recommends exposure levels not exceeding 4 pCi/L, or 0.04 WL.
workweek or work week [1]
a unit of time equal to the length of time a person "normally" works in a week. In many countries and in many industries, a specific number of hours, such as 35 or 40, constitutes a workweek either by law, by contract, or by custom, and workers who work more than that during a calendar week are entitled to be paid overtime wages.
workweek or work week [2]
a unit of time equal to the number of working or business days in a week (see above). During the nineteenth century the workweek was usually 6 days [3] but it is now 5 or 4 in most countries. Of course, individual workweeks are shorter if civil or religious holidays occur during the week.
wrench sizes
wrenches in the U.S. are sized in fractions of an inch or in multiples of 1/4 inch, while metric wrenches are sized in millimeters.
German abbreviation for Wassersäule, water column, seen in pressure measurements. See centimeter of water or millimeter of water.
an abbreviation for "weight by volume," a slightly confusing phrase used in chemistry and pharmacology to describe the concentration of a substance in a mixture or solution. The weight by volume is the mass (in grams) of the substance dissolved in or mixed with 100 milliliters of solution or mixture. For example, the concentration of fluoride in toothpaste is usually about 0.15% w/v, meaning that there is 0.15 gram of fluoride per 100 milliliters of toothpaste. Thus 1% w/v is equal to 1 gram per deciliter (g/dL) or 10 grams per liter (g/L).
an abbreviation for "by weight," used in chemistry and pharmacology to describe the concentration of a substance in a mixture or solution. Properly speaking, 2% w/w means that the mass of the substance is 2% of the total mass of the solution or mixture. The metric symbol g/g has the same meaning as w/w.
a unit of information in computer science, equal to 2 bytes or 16 bits. This name for the "double byte" was proposed by the American computer scientist Donald Knuth. It has achieved at least some use, but it is not well established.


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June 5, 2000