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什么是涡流?在生产中有何利弊?

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交变磁场中的导体内部将在垂直与磁力线的方向的截面上感应出闭合的环形电流,称涡流。 利用涡流原理可制成感应炉来冶炼金属,利用涡流可制成磁电式、感应式电工仪表, 电能表中的阻尼器也是利用涡流原理制成的;在电动机、变压器等设备中,由于涡流存在,将产生附加损耗,同时,磁场减弱,造成电气设备效率降低,使设备的容量不能充分利用。

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HowaFrenchmanisrevivingMcDonald’sinEurope A.WhenDenisHennequintookoverastheEuropeanbossofMcDonald’sinJanuary2004,theworld’sbiggestrestaurantchainwasshowingsignsofrecoveryinAmericaandAustralia,butsalesinEuropeweresluggishordeclining.OneexceptionwasFrance,whereMrHennequinhaddoneasterlingjobasheadofthegroup’sFrenchsubsidiarytosellmoreBigMacstohiscompatriots.Histaskwastoreplicatethissuccessinall41oftheEuropeancountrieswhereanti-globalisers’favouriteenemyoperates. B.SofarMrHennequinisdoingwell.LastyearEuropeansalesincreasedby5.8%andthenumberofcustomersby3.4%,thebestannualresultsinnearly15years.Europeaccountedfor36%ofthegroup’sprofitsandfor28%ofitssales.DecemberwasanespeciallygoodmonthascustomerstooktoseasonalmenuofferingsinFranceandBritain,andtoapromotioninGermanybasedonthegameofMonopoly. C.MrHennequin’srecipeforrevivalistobemoreopenabouthiscompany’soperations,tobe“locallyrelevant”,andtoimprovetheexperienceofvisitinghis6,400restaurants.McDonald’sisblamedformakingpeoplefat,exploitingworkers,treatinganimalscruelly,pollutingtheenvironmentandsimplyforbeingAmerican.MrHennequinsayshewantstoengageinadialoguewiththepublictoaddresstheseconcerns. D.Heintroduced“opendoor”visitordaysineachcountrywhichbecamehugelypopular.InPolandalonesome50,000visitorscametoMcDonald’sthroughthevisitors’programmelastyear.TheNutritionInformationInitiative,launchedlastyear,putdetailedlabelsonMcDonald’spackagingwithdataoncalories,protein,fat,carbohydratesandsaltcontent.Thedetailsarealsoprintedontray-liners. E.MrHennequinalsowantspeopletoknowthat“McJobs”,thelow-paidmenialjobsatMcDonald’srestaurants,aremuchbetterthanpeoplethink.Butsomeofhiseffortshavebackfired:lastyearhesparkedacontroversywiththeintroductionofa“McPassport”thatallowsMcDonald’semployeestoworkanywhereintheEuropeanUnion.PoliticiansaccusedthefirmofaploytomakecheaplabourfromeasternEuropemoreeasilyavailabletoMcDonald’smanagersacrossthecontinent. F.Tostayintouchwithlocalneedsandpreferences,McDonald’semployslocalbossesasmuchaspossible.ARussianisrunningMcDonald’sinRussia,thoughaSerbisinchargeofGermany.Thegroupbuysmainlyfromlocalsuppliers.Four-fifthsofitssuppliesinFrancecomefromlocalfarmers,forexample.(SomeoftheFrenchfarmerswhocampaignedagainstthecompanyinthelate1990ssubsequentlydiscoveredthatitwas,infact,buyingtheirproduce.)AndithirescelebritiessuchasHeidiKlum,aGermanmodel,aslocalbrandambassadors. 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