1. Thermal Expansion:

* Liquids expand when heated and contract when cooled. This is the fundamental principle behind the thermometer. The liquid inside the thermometer is carefully chosen for its predictable and relatively large expansion rate.

2. The Bulb:

* A bulb at the bottom of the thermometer contains the liquid. This bulb is designed to have a large surface area to efficiently absorb heat from the object being measured.

3. The Graduated Tube:

* The bulb is connected to a thin, graduated glass tube. This tube is sealed at the top to create a closed system. The graduations on the tube represent different temperature values.

4. Temperature Measurement:

* When the bulb is placed in contact with a substance, the liquid inside either expands or contracts. As the liquid expands, it rises up the graduated tube. Conversely, as the liquid contracts, it falls down the tube.

* The height of the liquid column in the tube corresponds to the temperature of the substance. The thermometer is calibrated so that the height of the liquid column directly translates to a specific temperature reading.

5. Common Liquid:

* Mercury was historically used in thermometers. It has a high expansion rate and a visible silvery color. However, due to its toxicity, mercury is being phased out in favor of alcohol-based liquids, which are safer and still offer good performance.

In Summary:

The liquid in a glass thermometer expands or contracts in response to changes in temperature, causing the liquid column to rise or fall within the graduated tube. The height of the liquid column directly indicates the temperature of the object being measured.