V

var: a unit of the reactive electric power delivered by an alternating current (AC) circuit. In an AC circuit, the electric potential or voltage (measured in volts) and the current (in amperes) alternate direction, varying smoothly according to sine curves. If there were no resistance in the circuit, the variations of the current would lag the variations in the voltage by 1/4 cycle, or 90°. In real circuits, the current can be separated into two parts: the "reactive" part, which lags the voltage by 90°, and a part in phase with the voltage. The reactive part does no useful work; it simply heats the conductor.The reactive power is the product of the voltage and the reactive part of the current. The name of the unit is an acronym for volt-ampere-reactive.
vara: a traditional unit of distance in Spanish- and Portuguese-speaking countries. The length of the vara varied (no pun intended), but in Spanish Latin America it was generally about 33 inches or a little longer. In Texas, where it was often used in land measurement, the vara was defined to equal exactly 33 1/3 inches, which is equivalent to 84.667 centimeters. In California, the vara was considered equal to 33 inches (83.82 centimeters) and in Mexico the former standard was 32.993 inches (83.802 centimeters). In southern South America the vara was usually about 34 inches (86.4 centimeters). The Spanish vara is shorter; it equals 32.908 inches or 83.587 centimeters. The Portuguese vara, on the other hand, is much longer; it equals 5 palmos or about 110 centimeters (43.3 inches). The word vara means a stick or pole.
vedro: a traditional Russian unit of volume equal to 100 charki. The vedro is about 12.30 liters (3.249 U.S. liquid gallons or 2.706 British imperial gallons). In Bulgaria, the vedro has also been used informally as a name for the dekaliter (exactly 10 liters or 2.642 U.S. liquid gallons). The word vedro means a bucket.
verchok: a traditional Russian unit of distance equal to 1/16 arshin, 1.75 inches or 4.445 centimeters. The plural is verchoki.
verge: an old name for the yard, taken from the Latin word virga for a twig or stick. In modern French, verge is the customary word for the English yard.
verst: a traditional Russian unit of distance, formerly used throughout eastern Europe. The verst equals 1500 arshin, which is 3500 feet, 0.662 88 mile, or 1066.8 meters. The unit is sometimes known by its German name, werst. In Finland it is called the virsta.
Vickers hardness number (HV or VHN) or Vickers pyramid number (VPN): a measure of the hardness of a metal introduced by Vickers in 1922. In the Vickers test (suitable for surface-hardened metals), a pyramidal diamond is pressed into the material being tested. The Vickers hardness is the amount of force applied to the diamond divided by the area of the indentation the diamond makes in the material; in practice the diagonal of the pyramidal indentation is measured and the result, computed in kilograms of force per square millimeter, is read from a table. (The reading should be stated as an empirical measurement, without units.) Up to about HV 500, the Vickers hardness is about 1.04 times the Brinell hardness.
viertel: a traditional unit of volume in several European countries. Oddly, although the name means "quarter" in German the viertel is not really 1/4 of any other unit. The Danish viertel equals 8 pots or about 7.74 liters (2.04 U.S. liquid gallons or 1.70 British imperial gallons). In Switzerland the viertel is 40 schoppen, which is exactly 15 liters (3.9626 U.S. liquid gallons or 3.3000 British imperial gallons).
virgate: an old English unit of land area equal to 1/4 hide. This was roughly 30 acres or 12 hectares.; a commercial rating of industrial lubricants. The grade the lubricant in centistokes.
vision: a term often used with a Snellen fraction in phrases such as "20/20 vision."
volt (V): the SI unit of electric potential. Separating electric charges creates potential energy, which can be measured in energy units such as joules. Electric potential is defined as the amount of potential energy present per unit of charge. Electric potential is measured in volts, with one volt representing a potential of one joule per coulomb of charge. The name of the unit honors the Italian scientist Count Alessandro Volta (1745-1827), the inventor of the first battery.
volt ampere (V ·A): a unit of electrical load used in power engineering. In an alternating current circuit, if the potential (measured in volts) and the current (measured in amperes) vary in phase with each other, then the power delivered (measured in watts) is the product of the potential and the current. In actual circuits, the potential and current are usually out of phase (see var, above), causing the device receiving the power to draw more current than its wattage requirements would suggest. The product of the potential and the actual current is the load, in volt amperes.
volume unit (vu): a unit used in telecommunications to describe the volume of a radio or television signal carrying complex information such as speech or music. These waves vary wildly in amplitude in order to represent the message they carry, so it is difficult to describe mathematically how large their "average" amplitude is. To solve this problem, the American Standards Association decided in 1951 to establish as a reference a signal which generates 1 milliwatt of power in a circuit having an impedance of 600 ohms. The volume of an incoming signal, in volume units, is equal to the number of decibels by which it exceeds this reference level. Thus the volume unit is (in a complicated way) a variation of the decibel.
v/v: an abbreviation for "by volume," used in chemistry and pharmacology to describe the concentration of a substance in a mixture or solution. Thus 2% v/v means that the volume of the substance is 2% of the total volume of the solution or mixture. See also w/v.