How the Galileo Thermometer Works.

The principle of how the thermometer actually measures temperature is based around the fact that the density of a substance (in this case water) changes in proportion to its temperature.

The device is made up of a glass tube filled with water, and inside are a number of bubbles of glass containing liqiud and air, tagged to each of the bubbles is a metal weight.  Now try and visualise each of the bubbles - they each have a different weight, so some are better at "floating" than others, the heavier ones are less bouyant than the lighter ones.

When the temperature outside the glass tube begins to change, the temperature is conducted through the tube and thus into the water within.  If the temperature rises (it gets hotter) we will see that some of the glass bubbles inside the tube begin to sink slowly - this can be recorded with a usb pen camera device for ease of use and spotting the movement.  This is because gravity pulls them down through the water.  When the water is cold, then gravity cannot pull the glass bubbles down because the molecules inside the water are packed more tightly together than when it is hot.

You can imagine that the water in the tube is like sand.  If you imagine the sandgrains spreading further apart the warmer it gets, you will see that the gap between each sand grain is getting wider and wider.  This means that objects can begin to fall through the sand.  Just like the glass bubbles can fall through the water.  Each of the glass bubbles is made accurately enough to stay bouyant until the gaps in the water are large enough to let it through and sink.

The temperature of the air around the tube is read on the thermometer as the one which is still floating, but is at the bottom of the floating stack.

Back to the Galileo Thermometer main page.