【正文】 不再轉動，因此，有必要阻止不同極成一直線。這使轉子朝著相吸的新極方向運動，完成一個完整的循環(huán)，或叫“一個周期”。隨著電流的變化，電子流以很高的速度不斷換向，換向間隔時間固定，目的是同時改變定、轉子的極以使轉子不斷旋轉。電磁感應的這種特性對于生產電能也是至關重要的。而根據(jù)電磁感應原理制成的發(fā)電機則能生產出大量便宜的電能以滿足世界上大部分的需求。法拉第發(fā)現(xiàn)反過來也行——磁場也能產生電流。由于電動勢由此而產生，發(fā)電機的功率以伏特計算，這是電動勢的度量單位。當然，導線移動速度越快，產生的電動勢就越強。一臺發(fā)電機中包括一個定子，即一塊不動的磁鐵，而轉子被置于磁鐵的南北極之間。每半圈電流就變換一次方向。轉子是由一臺渦輪機帶動的，它是一種帶大葉片一般由水或蒸汽驅動的的機器?，F(xiàn)代發(fā)電機的巨大外型和轉子的轉速向我們表明它能發(fā)出高壓電，高達五十萬伏的電力可通過高壓線輸送到各個變電站。在美國，家用電器通用電壓為110120伏，因為人們發(fā)現(xiàn)，高壓會引起致命的傷害事故。附錄2（原文）The property of electromagnetismOther scientists experimenting with electromagnetism found that the magnetic effect of an electric current could be strengthened by sending the current through a coil－a wire conductor twisted into a spiral shape．A greater number of turns of wire in the coil streng thens the magnetism，as does a stronger electric current．Wrapping the coil around a core of iron further increases the magnetism，because the iron itself bees magnetized．All these discoveries led the way to converting electromagnetic energy into motion．The great advantage of this energy conversion is that devices based on electromagnetism can be controlled simply by switching the current on or off．One such device is an electric motor，In which a bar known as an armature is placed between the two arms of horseshoe magnet－Magnetic poles are induced in the armature by sending a current through a coil wrapped around it．Magnetic force then causes it to move in the direction in which the unlike poles attract each other, Of course，if the poles of the armature reached a position directly opposite the unlike poles of the horseshoe magnet, the armature would bee locked and no further motion would be possible．It is necessary，therefore，to prevent the unlike poles from being aligned．In a direct current electric motor，this is done by a device called a mutator， which reverses the electron flow，changing the magnetic poles of the armature at each halfturn．This causes the armature to move on to the new poles of attraction，pleting a full rotation，or’cycle’．In an alternating current electric motor，the stationary portion is called a stator and the rotating portion，a rotor．with alternating current，the flow of electrons reverses at rapid intervals．The intervals are timed to change the stator and rotor poles simultaneously so that the rotor continues to move in a circle，or cycle．In the United States，alternating current electric motors are timed to plete 60cycles per second（abbreviated 60cps），but in Europe they are timed for 50cycles per second．The property of electromagnetism is also vital to the production of electrical power．Electricity from chemical action－the cell or battery－is suitable only for special and limited uses．Generators based on electromagnetism，however，produce sufficient cheap electricity to supply most of the world1s needs．Not long after the discovery that a magnetic field could be created by an electric current，the English scientist Michael Faraday discovered that the reverse was also truea current could be created by a magnetic field．When a closed loop of wire moves through a magnetic field，an electromotive force is created．This causes a current d electrons to flow through the wire and is the basis for generating electricity．Because it is electromotive force that is produced，the power of generators is described in terms of volts，tile units of measurement for emf ．To generate emf，the wire must cut the lines of force in the magnetic fiel