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职称英语理工类考试考前每日一练 Electdc Backpack Backpacks are convenient.They can hold your books，your lunch，and a change of clothes，leaving your hands free to do other things.Someday，if you don，t mind carrying a heavy load，your backpacks might also power your MP3 1 player，keep your cell phone running，and maybe even light your way home. Lawrence C.Rome and his colleagues from the University of Pennsylvania in Philadelphia2 and the Marine Biological Laboratory in Woods Hole3，Mass.4，have invented a backpack that makes electricity from energy produced while its wcarer walks.In military actions.searchandrescue operations，and scientific field studies，people rely increasingly on cell phones，global positioning system5 receivers，nightvision goggles，and other batterypowered devices to get around and do their work.The backpack，s electricitygenerating feature could dramatically reduce the amount of a wearer，s load now devoted to spare batteries，report Rome and his colleagues in the Sept.9 Science6. The backpack，s electricitycreating powers depend on springs used to hang a cloth pack from its metal frame.The frame sits against the wearer，s back，and the whole pack moves up and down as the person walks.A gear meehanism converts vertical movements of the pack to rotary motions of an electrieal generator，producing up to 7.4 watts. | 
