Core Principle: Thermal Expansion
Most common thermometers, such as the liquid-in-glass type, operate on the principle of thermal expansion. This physical property dictates that substances tend to expand in volume when heated and contract when cooled, due to changes in the kinetic energy and spacing of their constituent particles.
The Role of the Liquid and Tube
A thermometer contains a sealed narrow glass tube with a bulb at one end, filled with a temperature-sensitive liquid like colored alcohol or mercury. This liquid is specifically chosen because it expands and contracts uniformly and significantly over a broad range of temperatures, much more so than the glass casing.
Visualizing Temperature Change
When the thermometer encounters a warmer environment, the liquid in the bulb absorbs thermal energy and expands. Because the glass tube is extremely narrow (a capillary tube), even a small increase in the liquid's volume causes it to rise visibly up the tube. Conversely, in colder conditions, the liquid loses heat, contracts, and its level falls.
Calibration and Reading the Scale
The glass tube is marked with a calibrated scale (e.g., Celsius or Fahrenheit). These markings are set against known reference points, like the freezing and boiling points of water. By observing where the top of the liquid column aligns with the scale, one can accurately read the current temperature.