Oxford University Press's
Academic Insights for the Thinking World

# Physics Project Lab: How to make your own drinking bird

Over the next few weeks, Paul Gluck, co-author of Physics Project Lab, will be describing how to conduct various different Physics experiments. In his second post, Paul explains how to build your own drinking bird and study its behaviour in varying ways:

You may have seen the drinking bird toy in action. It dips its beak into a full glass of water in front of it, after which it swings to and fro for a while, returns to drink some more, and so on, seemingly forever. You can buy one on the internet for a few dollars, and perform with it a fascinating physics project.

But how does it work?

A dyed volatile liquid partially fills a tube fitted with glass bulbs at both ends. The lower end of the tube dips into the liquid in the bottom bulb, the body. The upper bulb, the head, holds a beak which serves two functions. First, it shifts the center of mass forward. Secondly, when the bird is horizontal its head dips into a beaker of liquid (usually water), so that the felt covering soaks up some of the liquid. As the moisture in the felt evaporates it cools the top bulb, and some of the vapor within it condenses, thereby reducing the vapor pressure of the internal liquid below that in the bottom bulb. As a result, liquid is forced upward into the head, moving the center of mass forward. The top-heavy bird tips forward and the beak dips into the water. As the bird tips forward,  the bottom end of the tube rises above the liquid surface in the bulb; vapor can bubble up from the bottom end of the tube to the top, displacing some liquid in the head, making it flow back to the bottom. The weight of the liquid in the bulb will restore the device to the vertical position, and so on, repeating the cycle of motion. The liquid within is warmed and cooled in each cycle. The cycle is maintained as long as there is water to wet the beak.

The rate of evaporation from the beak depends on the temperature and humidity of the surroundings. These parameters will influence the period of the motion. Forced convection will strongly enhance the evaporation and affect the period. Such enhancement will also be created by the air flow caused by the swinging motion of the bird.

Here are some suggestions for studying the behaviour of the swinging bird, at various degrees of sophistication.

Measure the period of motion of the bird and the evaporation rate, and relate the two to each other. You can do this also when water in the beaker is replaced by another liquid, say alcohol. To measure the evaporation rate the bird may be placed on a sensitive electronic balance, accurate to 0.001 g. A few drops of the external liquid may be applied to the felt of the head by a pipette. Measure the time variation of the mass of this liquid, and that of the period of motion, without replenishing the liquid when the bird bows into its horizontal position. Allow for the time spent in the horizontal position. Establish experimentally the time range for which the evaporation may be taken as constant.

Explore how forced convection, say from a small fan directed at the head, changes the rate of evaporation, and thereby the period of the motion.

The effects of humidity on the period may be observed as follows: build a transparent plexiglass container with a small opening. Place the bird inside. Vary the internal humidity by injecting controlled amounts of fine spray into the enclosed space. You can do this by using the atomizer of a perfume bottle.

By taking a video of the motion and analyzing it frame-by-frame using a frame grabber, measure the angle of inclination of the bird to the vertical as a function of time.

Do away altogether with the beaker of liquid in front of the bird and show that all it needs for oscillatory motion is the presence of a difference of temperature between the bottom and the top, a temperature gradient. To do this, paint the lower bulb and the tube black, and shine a flood lamp on them at controlled distances, while shielding the head, so as to create a temperature gradient between head and body. Such heating increases the vapor pressure within, causing liquid to be forced up into the head and making the toy dip, just as for the cooling of the head by evaporation. It will then be interesting to study how the time elapsed before the first swing and the period of motion are related to the effective surface being illuminated (how would you measure that?), and to the effective energy supplied to the bird which itself will depend on the lamp’s distance from the bird

There are many more topics that can be investigated. As one example, you could follow the time dependence of the head and stem temperatures in each cycle by means of tiny thermocouples, correlating these with the angular motion of the bird. Heat enters the tube and is transported to the head, and this will be reflected in a steady state temperature difference between the two. Both head and tube temperatures may vary during a cycle, and these variations can then be related to heat transfer from the surroundings and evaporation enhancement due to the convection generated by the swinging motion. But for this, and other more advanced topics, you would have to have access to a good physics laboratory, obtain guidance from a physicist, and be willing to learn some heat and thermodynamics as well as the mechanics of rotational motion, in addition to investing more time in the project.

Have you tried this experiment at home? Tell us how it went to get the chance to win a free copy of the Physics Project Lab book! We’ll pick our favourite descriptions on 9th January.

Featured image credit: ‘Drinking bird photo’ by Christopher Zurcher, CC-by-2.0, via Flickr