"Perpetual Motion" Clock Run by Changes of Temperature

From American Horologist magazine, April, 1936

"Perpetual Motion" Clock Run by Changes of Temperature

Here is the "perpetual motion" clock -no spring, no winding, no batteries nor current, no magnets nor selenium cells. Yet it runs! It is activated by changes of temperature. A variation of only one degree centigrade in the air temperature is enough to keep the clock going for 120 hours, and it varies less than a minute a year in accuracy. The motive element is contained in a U-shaped tube of Pyrex glass fixed in a drum. One end of the tube is exposed to room temperature, the other end is shielded by insulating material and a cap. Inside the tube, at the base of the "U," is mercury; above it, on each side, a liquid gas; and at the top of the tube, the saturated vapor of the gas. As the temperature rises, the vapor in the exposed side expands, pushing the mercury toward the insulated end, compressing the gas in the latter. The shift of mercury changes the center of gravity and the drum revolves slightly storing energy. As the temperature drops, the drum rocks back and builds more energy.