Backpacks are convenient. They can hold your books，your lunch，and achange of clothes，leaving your hands free to do other things.Someday，if youdon't mind carrying a heavy load，your backpacks might also power your MP31player，keep your cell phone running，and maybe even light your wayhome.
Lawrence C. Rome and his colleagues from the University of Pennsylvania in Philadelphia2 and the Marine Biological Laboratory in WoodsHole3，Mass.4，have invented a backpack that makes electricity from energyproduced while its wearer walks. In military actions，search-and-rescueoperations，and scientific field studies，people rely increasingly on cellphones，global positioning system (GPS) 5 receivers，night-vision goggles，andother battery powered devices to get around and do their work. The backpack’selectricity- generating feature could dramatically reduce the amount of awearer’s load now devoted to spare batteries，report Rome and his colleagues inthe Sept.9 Science6.
The backpack’s electricity-creating powers depend onsprings used to hang a cloth pack from its metal frame.The frame sits againstthe wearer’s back，and the whole pack moves up and down as the person walks.Agear mechanism converts vertical movements of the pack to rotary motions of anelectrical generator，producing up to 7.4 watts.
Unexpectedly，tests showedthat wearers of the new backpack alter their gaits in response to the pack’soscillations，so that they carry loads more comfortably and with less effortthan they do ordinary backpacks. Because of that surprising advantage，Rome plans to commercialize both electric and non-electric versionsof the backpack.
The backpack could be especially useful forsoldiers，scientists，mountaineers，and emergency workers who typically carryheavy backpacks.For the rest of us，power-generating backpacks could make itpossible to walk，play video games，watch TV，and listen to music，all at the sametime.Electricity-generating packs aren’t on the market yet，but if you do get one eventually，just make sure to look both ways before crossing thestreet!