Scavenging power from trees

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Engineers have run an electrical circuit using the power of tree, seemingly the first demonstration of its kind. The University of Washington researchers determined that bigleaf maples on the school's campus generate up to few hundred millivolts. (The current is not mentioned.) So they built a low-power sensing circuit that could scavenge enough juice from a tree to operate. From UWNews:
The tree-power phenomenon is different from the popular potato or lemon experiment, in which two different metals react with the food to create an electric potential difference that causes a current to flow.

"We specifically didn't want to confuse this effect with the potato effect, so we used the same metal for both electrodes," (electrical engineering professor Babak) Parviz said.

Tree power is unlikely to replace solar power for most applications, Parviz admits. But the system could provide a low-cost option for powering tree sensors that might be used to detect environmental conditions or forest fires. The electronic output could also be used to gauge a tree's health.

"It's not exactly established where these voltages come from. But there seems to be some signaling in trees, similar to what happens in the human body but with slower speed," Parviz said. "I'm interested in applying our results as a way of investigating what the tree is doing. When you go to the doctor, the first thing that they measure is your pulse. We don't really have something similar for trees."
"Electrical circuit runs entirely off power in trees"