The air thermometer was one of the earliest kinds of thermometer. To measure a sick person's temperature, there was a version made of glass, with a bulb to put in the mouth. The air thermometer has several advantages: air is cheap and doesn't have any bad properties (unlike mercury, which used to be common in thermometers but really is too poisonous), and air expands more than any liquid or solid.

We can make our own thermometer, from these materials:

• plastic flower tube with cap *
• thin straw *
• small amount of colored water *
• hot and iced water in containers **

• Fill the flower tube about 1/4 full with colored water.
• Place the cap on securely.
• Slide the straw through the hole in the cap, all the way down until it almost touches the bottom of the tube.
• This is the air thermometer. Note that it is important that the seals between cap and tube and between cap and straw stay intact, for the thermometer to work.

What is the sensitive part of the thermometer? To find out, try to heat only the water (by just immersing the very bottom). Also compare the behavior of your thermometer to another, which has a lot more water and a lot less air in it.

Interpret your results. What role do the different parts of the system (tube, cap, straw, water, coloring) play?

Here are some questions to discuss:

• What are the disadvantages of an air thermometer? Why don't we use them more often?
• How many strawfuls of water does it take to fill the flower tube? What role does this ratio of volumes play in how the air thermometer works?
• If we wanted a thermometer that made a bigger change with temperature (so that we could detect changes in temperature of a small fraction of a degree), how might we modify the design of the thermometer?

Check the box when you are done:   Science content: thermal expansion