W

watch [1]

a traditional unit of time, defined as the time a sentry stands watch or a ship's crew is on duty. On both land and sea, one watch is usually equal to 4 hours. 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 (8 hours). This use is being 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 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. There are differing estimates of this flow rate. One estimate is 14 pints per minute or 2520 gallons per day (this is equivalent to 6.530 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 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)

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 (and thus one kilowatt equals about 1.341 02 horsepower). 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 who built the first practical steam engines. See Power

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. See Energy

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 required to be precisely 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) in 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 10⁸maxwells, 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. 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. Christians and Moslems inherited the seven-day cycle from the Jewish religion. When the Emporer Constantine legalized Christianity in the Roman Empire, early in the fourth century CE, he added the week to the Roman calendar. 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. See Time.

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 ton of the solid material that would remain if all the water were removed.

wey

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.

wind chill index (WCI)

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 currently used by U.S. meteorologists, the wind chill index is computed from the

1. temperature T (in °F) and
2. wind speed V (in mi/hr) using the formula:
3. WCI = 0.0817(3.71 sqrt(V) + 5.81 - 0.25V)(T - 91.4) + 91.4.
4. The metric equivalent:
5. temperature in °C
6. V in km/hr, is:
7. WCI = 0.045(5.27 sqrt(V) + 10.45 - 0.28V)(T - 33) + 33.
8. For very low wind speeds, below 4 mi/hr or 6 km/hr, the WCI is actually higher than the air temperature, but for higher wind speeds it is lower than the air temperature. See Wind Chill Calculator

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.

wineglass

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.

wineglassful

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 0.60 U.S. cup.

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 varies with the computer system; 32 and 36 bits are typical values.

WS

German abbreviation for Wassersäule, water column, seen in pressure measurements. See centimeter of water or millimeter of water.

w/v

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).

w/w

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.