Dr. Sue Ann Bowling is an Associate Professor of Physics at the Institute. A modern Gallilean Thermometer. The expression of temperature in numbers seems natural today. But the idea of assigning numbers to temperatures is only a few centuries old. Early thermometers, along with most of those in home use today, were based on the same principle that assists in opening jar lids held under hot water.
Most substances expand when heated, but the expansion differs for different materials.
Glass expands only slightly when heated, but metal jar caps, mercury and alcohol expand much more for the same increase in temperature. The increase in volume of all of the fluid in the bulb has to be taken up in the thin channel of the thermometer stem, so a small change in temperature can be seen as a relatively large change in the position of the fluid in the tube. Although the bulb and stem thermometer is the common type today, it is not the only way to use the change of volume of a liquid to measure temperature, nor was it the first way. Gallileo invented a thermometer some 350 years ago that was based on measuring the change of density of a liquid, rather than the change of volume directly. Density and volume are closely related, since the density is just the mass of the liquid divided by its volume, and the mass does not change as it is heated.