Galileo's Thermometer

This has been known for a long time. About 400 years ago Galileo use this to develop a thermometer. He built a thermometer comprised of a number of small glass spheres with small tags on the bottom and placed in a tall cylindrical container filled with water. The water temperature is assumed to be close to the room air temperature. The water thermometer value is read as the number on the sphere tag which is at the bottom of the floating spheres. Select a temperature in the figure and see how the spheres move. Change the water temperature by selecting the temperature slider and moving it. Watch what happens to the spheres.

The reason they move as they do is that the effective density of the sphere with the 72 number on it is made equal to the density of water at 72 degrees. Remember the density of something is just the weight of it divided by the volume. One way to make these is to use a larger container of water with a better thermometer in it. Heat it up till it is 72 degrees. Them place the glass sphere with the 72 value tag into it. Add small weights to the sphere until it will just stay motionless in the water a few inches below the surface. It then has the effective density of the water at this temperature. The water is heated, or cooled, and the same procedure repeated for all the rest of the spheres.

All the spheres that have tags values higher than the water temperature will be near the surface while all those with numbers lower will be near the bottom of the cylinder. As the water temperature rises, the water density increases, and some of the spheres which could float in the higher density water now sink in the lighter density water.

I just purchased one of these and they are lovely to look at and very accurate to boot!

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