The devices produce electricity when a temperature difference exists between two ends of a circuit made from two types of metal. The company has packed 5,074 thermocouples (electricity-producing circuits) into the generator, and the device provides a relatively high voltage from a smaller temperature difference: At a difference of 5 degrees Celsius, the generator produces 3.1 volts at 36 microamps, yielding 110 microwatts of electrical power.
Thermoelectric generators are particularly useful for powering wearable devices because people aren't always in the light and don't always move, but as long as they're alive they generate body heat. The temperature difference between the skin and the surrounding air is about 15 degrees Kelvin, said Stark. Even within the body there is a useable temperature difference between the core and the surface, he said.