- 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 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 
- 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
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 
- another name for a shift. This use was
popularized in the U.S. by CNN Headline News and by the NBC television series
- water column (WC)
- a notation seen in pressure measurements. See inch
of water, centimeter of water, or millimeter
- 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 
(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
- watt (W) 
- 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
- watt (W) 
- 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
- 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
- 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
- week (wk)
- a traditional unit of time equal to seven days . 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:
calendar provides calendars for any month and year in the current (Gregorian)
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  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 ,
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
- 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,
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
- 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 
- 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 
- 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
- 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 
- 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 
- 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  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
- 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