Themodernageisanageofelectricity.Peoplearesousedtoelectriclightsradiotelevisionsandtelephonesthatitishardtoimaginewhatlifewouldbelikewithoutthem.Whenthereisapowerfailurepeoplegropeaboutinflickeringcandlelightcarshesitateinthestreetsbecausetherearenotrafficlightstoguidethemandfoodspoilsinsilentrefrigerators.

Yetpeoplebegantounderstandhowelectricityworksonlyalittlemorethantwocenturiesago.Naturehasapparentlybeenexperimentinginthisfieldformillionofyears.Scientistsarediscoveringmoreandmorethatthelivingworldmayholdmanyinterestingsecretsofelectricitythatcouldbenefithumanity.

Alllivingcellssendouttinypulsesofelectricity.Astheheartbeatsitsendsoutpulsesofrecord;theyformanelectrocardiogramwhichadoctorcanstudytodeterminehowwelltheheartisworking.Thebraintoosendsourbrainwavesofelectricitywhichcanberecordedinanelectroencephalogram.Theelectriccurrentsgeneratedbymostlivingcellsareextremelysmall，oftensosmallthatsensitiveinstrumentsareneededtorecordthem.Butinsomeanimalscertainmusclecellshavebecomesospecializedaselectricalgeneratorsthattheydonotworkasmusclecellsatall.Whenlargenumbersofthesecellsarelinkedtogethertheeffectscanbeastonishing.

Theelectriceelisanamazingstoragebattery.Itcansendajoltofasmuchaseighthundredvoltsofelectricitythroughthewaterinwhichitlives.(AnelectrichousecurrentisonlyonehundredandtwentyvoltsbuttwohundredandtwentyvoltsinChina.)Asmanyasfour-fifthsofallthecellsintheelectriceel'sbodyarespecializedforgeneratingelectricityandthestrengthoftheshockitcandelivercorrespondsroughlytolengthofitsbody

56.Electricitywasinvented______.

A.whenmanhadnocandles

B.about200yearsago

C.tobeoperatingcomputers.

D.byThomseEdison

57.Thefollowingthingscansendoutpulsesofelectricityexcept______.

A.electriceelsandhumanhearts.

B.Electricalgeneratorsandanimalmuscle.

C.Stonesanddrywood.

D.humanbrainandlivingcells.

58.Theelectriccurrentsendoutbyaneelcanbe

A.asmuc