From Popular Science: Kerosene lamps used in off-grid, rural areas are a major problem. They’re bad for people’s health and the environment’s. One startup’s solution is to tap another, greener resource, something we all have in abundance: gravity. The invention, GravityLight, does exactly what the name suggests: It keeps a light going through the power of gravity. As an attached weight falls, it pulls a cord through the center of the light, powering a dynamo. That dynamo converts the energy from the falling weight into power for the light. (It’s the same idea as a hand-cranked device, just more vertical.) The weight can be set in a few seconds, and as it slowly reaches Earth, enough energy is generated to keep a light working for 30 minutes. As long as it’s set every 30 minutes, it makes for a green, battery-free, continuous stream of light. Other, similar devices like battery chargers could be used through the same process, too.

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