Old principle, new application: Two London-based designers have invented an electric light called GravityLight for developing countries that uses gravity to generate light. The principle is simple. A strap runs through the mechanism of the LED light. Pulling the strap raises a bag filled with sand, soil or stones weighing about 9 kilograms (20 pounds). As the weight gradually descends, the energy thus released is converted to electricity that lights the lamp. Three seconds’ worth of effort gives light for about 25 minutes. Mass produced GravityLight will cost about six or seven US dollars a piece and help to replace the dangerous and environmentally harmful kerosene lamps currently used to light huts in many developing countries.