PR-82
Drinking bird, also known as dipping bird is actually a heat engine that mimics the motions of a bird drinking from a water source.
It is not just a toy. It also is a demonstration apparatus on liquid-vapor equilibrium and evaporation.
The bird is made of glass, and the lower bulb (body) is filled with a colored liquid.
The upper bulb (head) and the beak are covered in a felt-like material. The bird is typically decorated with eyes, a plastic top hat and feathers.
The whole setup pivots on an adjustable crosspiece attached to the neck.
The apparatus exploits a temperature difference to convert heat energy to a pressure difference within the device, and performs mechanical work.
The initial state of the system is a bird with a wet head oriented vertically.
When water evaporates from the felt on the head, it lowers the temperature of the glass head, causing the vapor in the head to condense. These together cause the pressure to drop in the head.
The higher vapor pressure in the warmer base pushes the liquid up the neck. As the liquid rises, the bird tips over.
With the tipping, the bottom end of the neck tube rises above the surface of the liquid and the warm vapor rises up the tube through this gap, displacing the liquid as it goes.
Liquid flows back to the bottom bulb and pressure equalizes between the top and bottom bulbs, thus restoring the bird to its vertical position.
The liquid in the bottom bulb is heated by ambient air, which is at a temperature slightly higher than the temperature of the bird's head.
If a glass of water is placed so that the beak dips into it on its descent, the bird will continue to absorb water and the cycle will continue as long as there is enough water in the glass to keep the head wet.