How It Works

How does a Galileo thermometer work?

The Galileo thermometer consists of a vertical glass tube, typically filled with water, and sealed glass bubbles containing coloured water or alcohol. Each bubble is also attached to a specific mass (labelled with the temperature it represents) to calibrate its density (the amount of mass in a given volume). The temperature can be read by interpreting the distribution of these bubbles. The principle of buoyancy states that if an object is less dense than a liquid, it floats; and if the object is denser than the liquid, it sinks.
When the temperature of the liquid in the glass tube begins to warm up, it expands; hence lowering the density of the liquid, as its mass now occupies a larger volume. The opposite occurs when the temperature cools (ie density of the liquid increases). Therefore, if a bubble becomes denser compared with the liquid, it sinks; and if less dense, it floats.

  • I’m skeptical that there’s any expansion of the liquids. We’re talking about glass tubes, here. I’m thinking that since there’s nowhere for the liquid to expand to, its pressure changes, and this causes the rising and falling of the bubbles. That could account for the same observed effect because pressure and volume are on the same side of the equation, PV=nRT