航空術(shù)語詞典Dictionary of Aeronautical Terms 下

時(shí)間：2011-03-11 23:18來源：藍(lán)天飛行翻譯 作者：航空 點(diǎn)擊：次

The walls are too thin to transfer much heat into the bottle by conduction, and almost no heat passes across the vacuum. A vacuum bottle is a form of Dewar flask. See Dewar flask.
vacuum capacitor (electrical component). A capacitor that uses two concentric tubes as the plates, or conductors, and a vacuum between the tubes as the dielectric. A vacuum capacitor has a higher breakdown voltage than a similar capacitor that uses air as its dielectric.
vacuum distillation. A method of distilling a liquid by boiling it in a vacuum. The material being distilled is placed in a container, and as much air as possible is pumped from it. Because of the low pressure, the liquid boils at a lower temperature which prevents the material being damaged by heat.
vacuum forming. A method of forming parts from thermoplastic resin by using the weight of the atmosphere to apply pressure to the softened material. A sheet of thermoplastic material is placed above a female die and heated to soften it. When it is at the proper temperature, a low pressure, or vacuum, is produced inside the die, and atmospheric pressure forces the softened material into the die.
vacuum metallizing. A method of depositing an extremely thin film of metal on a surface. A metallic vapor is formed by evaporating the metal in a high vacuum, or extremely low absolute pressure. This vapor is attracted to the surface to be metallized where it forms a microscopically thin coating.
vacuum pump. An air pump used to remove air from a container. There are two types of vacuum pumps: those that produce a very low absolute pressure (a high vacuum) with almost no flow of air, and those that produce less vacuum (a higher absolute pressure) but move a much larger volume of air.
vacuum tube (electronic component). An electron control valve that has most of the air removed from the glass or metal envelope in which the electrodes are mounted. After as much air is removed from the envelope as is practical, a “getter” is ignited inside the tube to combine with any oxygen left inside the envelope. See getter.

vacuum-tube voltmeter. See VTVM.
valence electrons. Electrons that spin around the nucleus of an atom in its outer shell. It is the number of valence electrons in an atom that determines its chemical and electrical characteristics, and it is these electrons that can be moved from one atom to another to produce electron, or current, flow.
All chemical elements have between one and eight valence electrons. Elements with between one and three valence electrons are good electrical conductors. Elements with five to eight valence electrons are electrical insulators, and elements with four valence electrons are called semiconductors.
valence shell. The outer shell of electrons spinning around the nucleus of an atom.
valve (electronic device). The British name for a vacuum tube.
valve (mechanical component). A device that regulates or controls the flow of a fluid, either a liquid or a gas. Fluid power systems use both flow control and pressure control valves. Aircraft pressurization systems control the cabin pressure with outflow valves, and the fuel-air charge enters the cylinder of a reciprocating engine through the intake valve, and the burned gases leave through the exhaust valve.

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