Prayer wheel also known as mani wheel is a cylinder used in Tibetan Buddhism, and other Hindu religions, externally inscribed with a mantra, frequently, “Om mani padme hum,” and contains scrolls on which this and other mantras as well as sacred texts are written.
Turning the wheel (clockwise, never counterclockwise except among the adherents of Bon) releases the power inherent in the texts and prayers. The origins may lie in the power gained from circumambulation in general, since until recently the use of the wheel in Tibet to bear burdens was regarded as wrong.
Prayer wheels vary in size from small (from 3 inches in height) to large (at least 20 feet), and may set in rows. Some Tibetans turn prayer wheels themselves while other wheels are turned by waterpower and electric motors
It has a chinese origin, around 400 c.e. They are used to gain good karma and purify bad karma.
There are many types of prayer wheel: Water Wheels, Fire wheels, Wind Wheels, Electric Dharma Wheels and Stationary Prayer Wheels. A.G.H.
Bowker, John, The Oxford Dictionary of World Religions, New York, Oxford University Press, 1997, p. 764