This Gadget Stumped Einstein, But It Won't Fool You
Well dressed, active, mysterious – your new drinking bird is the James Bond of science toys! Start off by setting a full glass of room-temperature water in front of the bird, then dip the beak into said water, and the bird takes over from there, bending over to drink as long at it can reach the water! But how? What makes it move just from getting wet? It’s a mystery that literally stumped some of the world’s greatest minds, and now you’ll know the answer.
Your drinking bird is a version of one patented in 1946 by Bell Labs chemist Miles Sullivan. Similar devices were demonstrated as early as the 1880s, and in the early 20th century, there was a Chinese version called the “Insatiable Bird.” Supposedly, Albert Einstein and his wife Elsa encountered the insatiable bird when they visited Shanghai in 1922; unwilling to cheat and take it apart, Einstein wasn’t able to figure out how the enigmatic avian worked after three and a half months of studying it. It also stumped president Herbert Hoover, a trained engineer.
So what makes it tick? Here’s a hint: ask yourself why it doesn’t work without the water–what does water on that beak do? It doesn’t soak through into the sealed bird; it’s not really pushing the bird backwards with any great force. But what it is doing is evaporating. How would that make the bird move? First we need to understand how your bird is made. It’s a sealed system, with a small amount of the volatile substance methylene chloride inside. The MC exists inside the bird in two states: in the bottom it’s mostly liquid, with some vapor at the top of the bulb; in the top bulb (the head) it’s all vapor. The way the bird is constructed, the liquid on the bottom keeps the two sets of vapors, which are normally the same pressure, separate.
This is where evaporation comes in. The evaporating water cools the top of the bird by about .3 °Celsius, causing the methylene chloride vapor inside the top bulb to condense, lowering its pressure. Since the pressure from the vapor in the bottom is now higher, it pushes the liquid up the tube, changing the center of gravity and causing the bird to tip over. When the bird tips and liquid fills the top bulb, suddenly there is a channel for the vapor in the top and the vapor in the bottom to connect, and voilà! The pressures equalize, causing the liquid to drop back down into the bottom of the tube, and the bird to flip its head back up. The process then begins again, and will keep going as long as there is water to coat the bird’s beak.
This is essentially a reflection of the Ideal Gas Law, expressed in the equation PV=nRT. P, V, and T are the pressure, volume, and temperature, n is the amount of the gas, and R is the Ideal Gas Constant. What it essentially tells you is that as pressure increases, temperature increases and vice-versa, as temperature decreases, pressure will also decrease – like in your bird! Because the drinking bird uses changing temperatures to do mechanical work, it’s considered a “heat engine.” This means it is related to other mechanical engines powered by expanding gases, including steam turbines and the internal combustion engine in your car.
Now that you know that it’s the water evaporating that makes the bird work, you can try some experiments on your own: how could you speed up (or slow down?) the time it takes the bird to complete one cycle? What would happen if instead of water, you dipped the beak in a liquid with a different rate of evaporation? (hint: try alcohol, if you’re of appropriate age) Are there other ways to heat or cool just one of the bird’s bulbs, causing it to drink?
Everyone should have a friend who wears a top hat – and now you’ve got one that also demonstrates important scientific and engineering principles! Looks like ZZ Top was right, everyone’s crazy about a sharp dressed…bird.