What principle do glass stem thermometers operate on?

Glass stem thermometers operate on the principle of expansion and contraction of a fluid, typically mercury or colored alcohol, within a narrow glass tube. As the temperature increases, the fluid expands, rising in the tube to indicate the temperature on a calibrated scale. Conversely, when the temperature decreases, the fluid contracts and falls back down the tube. This clear relationship between the temperature and the volume of the fluid allows for accurate temperature measurement.

The design leverages the predictable behavior of liquids when subjected to temperature changes, making it a reliable method to gauge temperature in various environments. Understanding thermal expansion is fundamental in thermometry, as it is the key principle that enables these devices to function effectively.