Drinking bird

Drinking birds, also known as insatiable birdies, dunking birds, drinky birds, water birds, dipping birds, and “Sippy Chickens” are toy heat engines that mimic the motions of a bird drinking from a water source. They are sometimes incorrectly considered examples of a perpetual motion device. A drinking bird consists of two glass bulbs joined by a glass tube (the bird's neck/body). The tube extends nearly all the way into the bottom bulb, and attaches to the top bulb but does not extend into it. The space inside the bird contains a fluid, usually colored for visibility. (This dye might fade when exposed to light, with the rate depending on the dye/color). The fluid is typically dichloromethane (DCM), also known as methylene chloride. Earlier versions contained trichlorofluoromethane. Miles V. Sullivan's 1945 patent suggested ether, alcohol, carbon tetrachloride, or chloroform. Air is removed from the apparatus during manufacture, so the space inside the body is filled by vapor evaporated from the fluid. The upper bulb has a "beak" attached which, along with the head, is covered in a felt-like material. The bird is typically decorated with paper eyes, a plastic top hat, and one or more tail feathers. The whole device pivots on a crosspiece attached to the body. The drinking bird is a heat engine that exploits a temperature difference to convert heat energy to a pressure difference within the device, and performs mechanical work. Like all heat engines, the drinking bird works through a thermodynamic cycle. The initial state of the system is a bird with a wet head oriented vertically. The process operates as follows: The water evaporates from the felt on the head. Evaporation lowers the temperature of the glass head (heat of vaporization). The temperature decrease causes some of the dichloromethane vapor in the head to condense. The lower temperature and condensation together cause the pressure to drop in the head (governed by Equations of state). The higher vapor pressure in the warmer base pushes the liquid up the neck.