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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A1. Telegraphy, or pure continuous wave A2. Modulated telegraphy A3. Telephony A4. Facsimile A5. Television electromagnetic induction. The transfer of electrical energy from one conductor to another that is not electrically connected. This transfer is done by a changing magnetic field produced by current flowing in one conductor cutting across the other conductor. As the lines of flux expand and collapse across the conductor, a voltage is induced in it. electromagnetic interference. See EMI. electromagnetic radiation. One method by which energy is transmitted from one location to another. Radio and television signals, for example, reach us by electromagnetic radiation. When electric and magnetic fields oscillate back and forth at a high enough frequency, they extend out into space in the form of waves. These waves may be absorbed (received) at some distant point. Very little energy transmitted by electromagnetic radiation is lost, or used up, by the space between the point from which it is sent and the point at which it is received. Energy from the sun reaches the earth by electromagnetic radiation, and only a very small amount of this energy is lost in the empty space between the sun and the earth. electromagnetic vibrator. A device that interrupts a flow of direct current and changes it into pulsating DC. An electromagnetic vibrator consists of an electrical relay whose contacts are held closed by a spring and opened by the pull of the electromagnetic coil. The contacts and coil are connected in series across a source of direct current. When current flows through the coil, the magnetic field pulls the contacts open and stops the current. When the current stops flowing, the spring closes the contacts and current begins to flow again. The contacts vibrate between open and closed as long as the vibrator is connected to a source of direct current. electromagnetic waves. Vibrations of electric and magnetic fields that move at the speed of light. Electromagnetic waves radiate out at right angles to the direction of their wave motion. electromagnetism. The magnetic field surrounding a current-carrying conductor. The strength of the field is determined by the amount of current flowing in the conductor. electromechanical frequency meter. A type of instrument that uses the resonant frequency of a vibrating metal reed to measure the frequency of alternating current. Metal reeds having different lengths are vibrated by electromagnetism produced by the alternating current. The reed whose resonant frequency is the same as that of the AC will vibrate with the greatest amplitude, and the resonant frequency of this reed is shown on a calibrated scale on the face of the instrument. An electromechanical frequency meter is also called a vibrating-reed frequency meter. electromotive force. See EMF. |
