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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hairline crack. A tiny crack visible on the surface of a piece of material. hairspring. A flat, spirally wound spring used to provide a restraint for rotary motion of a shaft. Hairsprings are used in D’Arsonval-type electrical instruments to provide a calibrated restraint for the moving coil and to carry current into and out of the coil. Hairsprings are also used in spring-powered clocks and watches to regulate the movement of the balance wheel. HAL (height above landing) (air traffic control). The height above a designated helicopter landing area used for helicopter instrument approach procedures. halation (cathode-ray tube). A form of distortion that causes a blurred image of the trace on a cathode-ray display tube. The blurring is caused by reflection from the back of the fluorescent coating when the coating is too thick. half-duplex operation (communications). A type of communications in which signals can be sent in either direction, one direction at a time, but cannot be sent in both directions at the same time. half hitch. A type of knot used for lacing wire bundles together. The bundle is tied with two half hitches secured with a square knot. half-life. A measure of the rate of decay of radioactive material. In one half-life, the material decays, or loses one half of its radioactivity. In the next half-life interval, the material loses one half of that which is left. The half-life of various types of radioactive materials varies from a few microseconds (millionths of a second) up to billions of years. half-round file. A metalworking or woodworking file that has one flat side. The other side is convex—thicker in the center than at the edges. half-sectional view (technical illustration). A view of a component in which one half of it is cut away to show the inside. The other half is not cut away, and it shows the outside. half view (aircraft drawing). A mechanical drawing used to show a symmetrical object. The center line of the object is shown, and break lines indicate that only one half of the object is shown. half-wave antenna (radio antenna). A radio antenna whose electrical length is one half of the wavelength of the frequency for which the antenna is tuned. half-wave rectifier (electrical circuit). A form of electrical circuit that changes alternating current into pulsating direct current. One rectifier, either a semiconductor diode or a diode electron tube, is used to block the flow of one half of the AC wave while allowing the other half to pass through. Half-wave rectifiers are inefficient and do not produce smooth direct current. However, they are used where economy of components is important and pulsations in the output are of no concern. Hall-effect generator (electronic device). An electronic device which uses a thin wafer of semiconductor material to measure the intensity of a magnetic field. Control current flowing through the generator produces a Hall-effect voltage whose direction is perpendicular to the current, and whose intensity is proportional to both the control current and the strength of a magnetic field. The direction of the magnetic field passing through the generator is perpendicular to both the current and the voltage. |
