# The drinking bird toy heat engine Patented in 1945 by Miles V. Sullivan, the drinking bird is a toy heat engine perfect for your desk or for use at a STEM event to demonstrate some fundamental concepts in thermodynamics/physical chemistry. The toy is a sealed system consisting of two glass bulbs joined by a glass tube which extends into the bottom bulb so as to be positioned below the level of a pool of liquid. The top bulb is often covered in felt and a 'beak' to allow it to absorb water. There is a pivot somewhere along the tube so that the toy can rotate dependent on where the liquid is inside. The chamber is typically filled with dichloromethane[^1] (CAS 75-09-2), which is colourless but usually dyed when used in these birds. The quantity of vaporising medium should be in excess of the amount necessary to maintain vapour saturation at room temperature. Air is removed from the system so the internal headspace can form an equilibrium with $CH_2Cl_2$ vapour. [Read more (wiki).](https://en.wikipedia.org/wiki/Drinking_bird) [^1]:Dichloromethane is chosen due to its volatility (atmospheric boiling point of 39.6ºC). [[US2402463.pdf|Patent US2402463A ]] suggests that previous iterations contained "ether, alcohol, carbon tetrachloride or chloroform". ## Drinking cycle 1. Water evaporates from the felt on the head. 2. Evaporation draws heat and lowers the temperature of the vapour in the top glass bulb. 3. Low temperature in the top bulb causes vapour to condense and system pressure falls. 4. The higher vapour pressure in the base pushes liquid up the tube to the top bulb. 5. As the liquid rises up the tube the apparatus becomes top heavy and tips over. 6. As it tips, the bottom end of the tube rises above the level of liquid in the bottom bulb. 7. A slug of warm vapour rises up the tube, displacing liquid as it goes. 8. Liquid flows back to the bottom bulb and the pressure equalises between the bulbs. 9. Liquid in the bottom bulb is heated by ambient air, which is slightly higher than at the top bulb. 10. The weight of liquid now in the bottom bulb restores the bird to a vertical position. ## Physical and chemical principles for STEM - The dichloromethane with a low boiling point (39.6ºC) - as the drinking bird is first evacuated, partially filled and sealed, the pressure and thus the boiling point in the drinking bird will be different), gives the heat engine the ability to extract motion from low temperatures. - The combined gas law - establishes a proportional relationship between temperature and pressure exerted by a gas in a constant volume. - The ideal gas law - establishes a proportional relationship between number of gas particles and pressure in a constant volume. - The Maxwell–Boltzmann distribution - establishes that molecules in a given space at a given temperature vary in energy level, and therefore can exist in multiple phases (solid/liquid/gas) at a single temperature. - Heat of vaporisation (or condensation), which establishes that substances absorb (or release) energy when changing state. - Capillary action of the wicking felt. - Wet-bulb temperature - the temperature difference between the head and body depends on the relative humidity of the air.