Kid's Activity: Galileo Thermometer

by Melissa Jaramillo and Julie Snyder

An inventor is someone that uses his or her imagination and creative thinking to create something useful for the first time. These people pay attention to all that is around them. They also ask questions and keep on doing so until they can find their answer! All of us have the ability to be inventors and leave our own mark on history!

Galileo Galilei was an inventor/scientist that lived over 400 years ago. Despite the many years that have past however you are probably familiar with many of his discoveries and inventions. There is one that we use each and every day! It gives us an idea of whether it is cold or hot, what kind of clothes we should wear, and if we can go out and play. Can you guess what it might be?

A thermometer! That's right!

Thermometers measure temperature and tell us how hot or cold something is. We use them for the weather, when we are cooking, or sometimes you may have used a thermometer when you were sick to see if you had a fever. Galileo's thermometer is the kind that we use to tell us how warm the air is around us -- either indoors or out. It is very pretty to look at and rather simple to read.

The Galileo Thermometer probably appears much different than other thermometers you are familiar with. Take a look at the image on the left. You see the funny looking cylinder filled with colorful glass balls (spheres). Can you see some of the spheres are floating, staying near the top of the water? Others are sinking, moving towards the bottom of the container. Certainly it is fascinating to look at but how is it able to tell us how warm it is? To understand we will have to explore an intriguing scientific principle.

Galileo discovered that the density of liquids changes slightly with temperature. The density of a liquid indicates how heavy it is. As the temperature increases the molecules become more active and push apart from each other. Molecules are tiny particles that make up an object or substance. As the temperature rises the density of the liquid goes down. This is the key to how the Galileo thermometer works!

Let's do an experiment to illustrate this principle. Gather your materials:
• Wide mouth quart jar or other clear container
• Small glass bottle with tight lid
• Eyedropper
• Food coloring (optional)
• Thermometer and ruler(optional)
• Hot and cold water

Using an eyedropper, fill the small glass bottle about half way with water. This will be your "floating sphere." It must be calibrated or made to have a mass more than hot water, but less than cold water. To calibrate means that we will continue to adjust until it reaches a perfect level spot. When the bottle weighs less than the water, it will float. Can you guess what would happen if it weighs more?

Fill the small bottle about half full of water, add a couple drops of food coloring and put the top on tightly. Now test how it reacts when placed in the water. This is its buoyancy. It should sink slowly when placed in a quart jar of hot water. If it floats, add a few more drops of water. If it sinks quickly, remove a few drops.

Now fill the quart jar with very cold water. Does the small bottle float? If not, remove a couple more drops from it. (If your child will find this step tedious, calibrate in advance.) Can you mix warm and cold water until the bottle is suspended halfway from the bottom? Allow your child to try a variety of temperatures of water in the jar. Using the thermometer and a ruler you can introduce graphing by plotting the temperature of water against the height of the small bottle.

Look at the first image in this picture. Can you compare, based on your own experiment, if the water is cold, warm, or hot? In this picture the density of the small bottle equals the density of warm water, neither sinking nor floating. Its mass is less than cold water, so it floats. Since the small bottle has a density greater than hot water, it sinks. Amazing, isn't it?

Who would have thought that sometimes an object would sink and sometimes it would float, just because the water was warmer or colder? The Galileo Thermometer is based on this principle. Now it will be easy to apply what you have learned and understand this thermometer's function! It's a simple, fairly accurate thermometer, consisting of a sealed glass tube that is filled with liquid and several floating bubbles. The bubbles are glass spheres (balls) filled with a colored liquid mixture.

Each bubble has a little metal tag that indicates a temperature. The weights of the tags vary slightly; the higher the temperature, the less its tag weighs. Each bubble is calibrated to exactly the same density (the ratio of mass to volume) by adding fluid. This makes each different temperature tag and bubble combination have a slightly different density. As the temperature of the air outside the thermometer changes, so does the temperature of the liquid surrounding the bubbles. As the temperature of the liquid changes, it either expands or contracts, changing its density slightly. So, at any given density, some of the bubbles will float and others will sink.

Let's say there are five colored bubbles in the thermometer:

  • A red bubble calibrated to 64 degrees
  • A green bubble calibrated to 68 degrees
  • A blue bubble calibrated to 72 degrees
  • An orange bubble calibrated to 76 degrees
  • A purple bubble calibrated to 80 degrees

Remember, that calibrated in this example means to make it level at the center at that particular temperature. For instance, if you spy the blue bubble neither floating at the top nor sinking, but balanced, you would be able to tell it was 72 degrees! Can you guess which is the densest (heaviest)? Did you say the red one? Good for you! That's correct! To indicate a cooler temperature, it must not float until the surrounding liquid is as dense as it. The colder the liquid gets, the denser it becomes. See if you can determine the room temperature this thermometer is indicating: The purple sphere (80°) is floating on the top. The red (64°), green (68°) and blue (72°) have sunk to the bottom. The orange bubble (76°) is almost floating. It sinks slightly so that it is floating just below the purple and red bubbles -- indicating the room's temperature is 76°!

While today, most Galileo thermometers are used purely for decoration they still offer a useful guide to judging the temperature of the air around us. Don't forget the most valuable lessons offered by all inventors -- ask questions, explore, and always seek to learn more! Who knows! Perhaps 400 years from now a kid will be studying one of your inventions!

Julie Snyder is a mom of six, interested in kids, pregnancy, birth, people and lives in the outlying Seattle area. Melissa Jaramillo is mom to many. She's passionate about building, encouraging, and strengthening families on this adventure known as parenthood!

Copyright © Melissa Jaramillo and Julie Snyder. Permission to republish granted to, LLC.