时间：2011-03-11 23:13来源：蓝天飞行翻译 作者：admin 点击：次
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lead of a screw thread. The linear distance from the apex of one screw thread to the apex of the next thread. Lead is also the distance the screw advances in one complete rotation. leaf brake (sheet metal shop tool). A large bending tool used to make straight bends across a sheet of metal. The metal to be bent is clamped between a flat lower jaw and an upper jaw fitted with a radius bar having the proper bend radius. A heavy leaf, mounted on a hinge, folds up to bend the metal over the radius bar. A leaf brake is also called a cornice brake. leaf spring. A flat spring hinged at one end and arched in the center. The load is applied to the center of the arch and is absorbed as the spring alternately straightens out and returns to its arched shape. leakage (electrical). Electrical current that passes through an insulator which is supposed to stop all flow. The amount of leakage is determined by the dielectric strength of the insulator and the amount of voltage, or electrical pressure, across the insulator. leakage current (semiconductor characteristic). The flow of minority carriers in a semiconductor device when the junction is reverse-biased. Both high voltage and high temperature cause electrons to break out of their bonds and leave holes. These electrons and holes are attracted to the junction, where they recombine and cause a small amount of current to flow. Leakage current increases as the temperature of the device increases. leakage flux (magnetism). Magnetic flux that does not pass directly between the poles of a magnet. Leakage flux does no useful work. lean flameout (gas turbine engine operation). A condition in which the flame inside a gas turbine engine goes out because there is not enough fuel for the amount of air flowing through the engine. Turbine engine fuel controls are designed to measure the air flowing through the engine and adjust the fuel flow so a lean flameout, or lean die-out, cannot occur. A lean flameout is also called a lean die-out. lean mixture. A fuel-air mixture that contains more than 15 parts of air to 1 part of fuel, by weight. least significant bit. See LSB. least significant digit. See LSD. Leclanche cell (batteries). Another name for a carbon-zinc cell. An ordinary carbon-zinc flashlight battery is an example of a Leclanche cell. The negative terminal is the zinc can that forms the outside shell, and the positive terminal is a carbon rod supported inside the can by the electrolyte. The electrolyte is a moist paste of sal ammoniac (ammonium chloride), with manganese dioxide mixed in it to absorb the hydrogen bubbles which form on the carbon rod as the battery is used. LED (light emitting diode). An optoelectronic device that emits visible light when electrons pass through it. P and N impurities are mixed with such chemical compounds as indium phosphide, gallium arsenide, and zinc selenide to form a P-N junction. When the P-N junction is reverse-biased (the negative terminal of the voltage source is connected to the P-material), no electrons flow, and the LED remains dark. But when the diode is forward-biased (the negative terminal of the voltage source connected to the N-material), electrons flow through the LED, and it emits light. |
