telephones课件
日期：2010-01-14 01:49
heavy，Welcometojoinus!NextQuestionsabouttelephonesLesson91Lesson91Howdoesatelephonework?Thesoundwavesofyourvoicearechangedintoelectricalsignals，andexpensive返回目录Lesson91Whyarelightsignalsbetterthanelectricalsignals?返回目录Becauselightsignalscantravelalongerwaywithoutgettingweaker，thesignalissentdownapairofwirestothereceivingphoneThenitisturnedintosoundagain返回目录Lesson91Whyweretelephonesexpensiveatthebeginning?Becauseitwasdifficultforpeopletoheareachotheroverlongdistances;andthewireswerethick，andtheirsignalisclearertooLesson91Whatisamobilephone?AmobilephoneisasmarttelephonewhichhasnowiresandcanbecarriedinyourpocketwhereveryougoLesson91Whatisthedifferencebetweenamobilephoneandanordinaryphone?First:Amobilephonehasnowires;Second:Youcantakeitwithyouwhereveryougo;Inaword:AmobilephoneisveryconvenienttouseLesson91Whatisacordlessphone?AcordlessphonealsohasnowiresItssignalcanbesentorreceivedinalimiteddistanceItisoftenusedintheofficeorthefamily返回目录Lesson91Ifyou，
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