Charging your cell phone might soon be as simple as taking a walk around the block. Scientists have created a new machine that harvests energy from the movement of the knees while walking. Six volunteers wore the devices on their legs while strolling on a treadmill and were able to produce about five watts of electrical power each. That's enough energy to run 10 cell phones simultaneously. The apparatus captures the energy of a person's movement by coupling an electrical generator to knee motion.
When the wearer extends the knee, a gear on the device turns and spins the generator, which builds up energy. When the knee is flexed, the device turns off so as not to tax the wearer. The machine senses motion and knows when to activate itself. The design allows a person to use the tool without expending much additional energy beyond the normal demands of walking. The researchers measured the volunteers' oxygen intake and carbon-dioxide output to make sure it wasn't strenuous to use.
Work on a similar device built into a backpack was announced in 2005.
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