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New Watch Runs on Body Heat

February 8, 1999

A Japanese watch manufacturer has come out with a new wristwatch that generates its own power by utilizing the difference between ambient and body temperatures. The watch, which went on sale in late 1998, is fairly expensive, since it is a new technology. Even so, the manufacturer boasts confidently that "this is the first step toward realizing the technology's full potential. The real significance here is that we've created a commercial product that uses this new technology."

Power Generated Using Differences in Temperature
The principle behind this new watch's power generation has long been known. It was discovered by a German physicist named Thomas Johann Seebeck (1770-1831), and is therefore named the Seebeck effect. Basically, if two different types of metal are joined together in the shape of a ring, and if the two junctions are of different temperatures, electrons will flow from the warmer side to the cooler side, forming a kind of a generator. This principle has already been used to generate power for spacecraft such as planetary explorers, and research is under way to use it to produce electricity from waste heat generated by factories and power plants.

The material used to generate power for this new watch is a type of semiconductor in the shape of a rod. Power is generated when the ends of the rod are of different temperatures. A temperature difference of one degree centigrade generates 0.2 millivolts of electricity per rod; more electricity can be obtained when rods are strung together in alternating fashion.

The semiconductors used in this new watch are 0.08 mm on each side and 0.6 mm long; this allows 1,000 of them to be strung together and sandwiched between the watch face and back. With this construction, a difference of 10 degrees centigrade between body and ambient temperatures generates about 1.5 volts, the same as an ordinary dry cell battery.

Third Type of Self-Powered Wristwatch
There are already two kinds of self-powered watches: ones that generate power by using the natural movements of a person's arm to oscillate a power-producing mechanism, and ones that use natural or artificial light to generate electricity. This new type of watch uses a third energy source: heat. This third type of power generation is more suitable than movement or light for people who do not move around much and who spend significant amounts of time working in dimly lit spaces. The commercial production of the watch represents the culmination of three years of continuous research and development.

Since the power generation mechanism relies on the difference between the temperatures at the watch back (next to the wearer's arm) and the watch face, the battery will not charge if the watch is not worn for extended periods of time or if the wearer is in a warm climate where there is not much difference between body and ambient temperature. For this reason, the battery is designed to power the watch for 10 months once it is fully charged.

Efficiency Could Exceed Light Power in the Future
The watch is designed to conserve energy if power is not being generated. First, the second hand will stop moving; then, if power is not generated for three days, all hands will stop. When the watch is in power conservation mode, the push of a button will cause the hands to display the proper time.

This is the world's first heat-powered watch, but the company's Japanese rivals are hard at work to produce their own versions. The low energy output and high cost of this technology, however, are hampering further development. At present, the timepiece sells for 300,000 yen (2,609 U.S. dollars at 115 yen to the dollar)--quite expensive for a watch. In spite of this the manufacturer is confident about the commercialization of this new technology: "It's possible that in the future the technology's energy conversion efficiency will surpass that of light power, and prices could come down if the product becomes popular."

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Trends in JapanEdited by Japan Echo Inc. based on domestic Japanese news sources. Articles presented here are offered for reference purposes and do not necessarily represent the policy or views of the Japanese Government.

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