# How the drinking bird works

I am sure that most people have seen a variant of the drinking bird toy that seems to be able to go on indefinitely with no seeming source of energy. Bill the Engineering Guy looks at the origins of this ingenious toy and explains how it works.

(Via Mark Frauenfelder)

1. DonDueed says

Very well done, thorough explanation.
It must have taken a lot of experimentation to get the design so delicately tuned.

2. jrkrideau says

And I always thought it was evaporation. No wonder I only got out of Gr. 13 Physics by the skin of my teeth.

3. Rob Grigjanis says

jrkrideau @2: Evaporation is crucial. But if it stumped Einstein, I wouldn’t worry too much.

4. robert79 says

In the example where they shine a light on the bird, I can see where the energy is coming from. However, during normal operation it appears that the bird is doing work by extracting energy from the ambient temperature, which sounds like it violates the second law of thermodynamics.

Of course, even without the bird, the beaker of evaporating water is also doing work by extracting energy from the surrounding temperature, as the evaporated water is pushing against the atmosphere. I’m guessing there is some entropy involved in the evaporation process which balances the books somehow? So the bird is getting its energy through a temperature difference it creates by speeding up the evaporation process.

5. Rob Grigjanis says

robert79 @4: Evaporation of water from the head causes cooling. That will continue as long as the air isn’t saturated with water vapour.

When evaporation occurs, the energy removed from the vaporized liquid, will reduce the temperature of the liquid resulting in evaporative cooling.

6. Mano Singham says

robert79,

There is nothing special about the ambient temperature when its comes to the Second Law. All that the law requires is that the heat engine have a heat sink that is at lower temperature that the one from which it draws heat, and into which it can dump the heat that is not converted to work. In this case, the evaporation in the head creates a lower temperature region there.

Of cours, the excess heat eventually ends up in the room again, which seems like it creates a problem. But that is because the system is so small that you can create a small temperature gradient between the top and bottom. If you scaled this up in a sealed room, the room would eventually reach 100% humidity and the process would stop.

The Second Law is very unforgiving. It allows for no violations. As Arthur Eddington said:

“If someone points out to you that your pet theory of the universe is in disagreement with Maxwell’s equations — then so much the worse for Maxwell’s equations. If it is found to be contradicted by observation — well, these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.”

7. busterggi says

My amazment over how this works is second only to my joy that these are still being made.