The correct option is **C. Liquid-in-glass thermometer**. ### Explanation of the Correct Answer A liquid-in-glass thermometer operates on the principle of thermal expansion. This type of thermometer contains a liquid, typically mercury or colored alcohol, sealed within a glass tube. As the temperature increases, the liquid expands and rises in the tube. Conversely, when the temperature decreases, the liquid contracts and falls back down the tube. The height of the liquid column corresponds to the temperature, which can be read against a calibrated scale marked on the glass. #### Step-by-Step Explanation: 1. **Principle of Thermal Expansion**: - All liquids expand when heated and contract when cooled. This property is fundamental to the operation of a liquid-in-glass thermometer. The amount of expansion is relatively uniform over a range of temperatures, making it a reliable method for measuring temperature. 2. **Construction**: - The thermometer consists of a narrow glass tube with a bulb at one end filled with the liquid (mercury or alcohol). The other end of the tube is open to the atmosphere or sealed, depending on the design. 3. **Calibration**: - The thermometer is calibrated to a specific scale (Celsius, Fahrenheit, or Kelvin). The scale is marked on the glass tube, allowing users to read the temperature directly from the height of the liquid column. 4. **Reading the Temperature**: - To read the temperature, you simply look at the level of the liquid in the tube against the scale. The position of the liquid indicates the temperature of the environment or the substance being measured. ### Why the Other Options are Incorrect: - **A. Bimetallic thermometer**: - A bimetallic thermometer uses two different metals bonded together that expand at different rates when heated. This differential expansion causes the metal strip to bend, which moves a pointer on a dial to indicate temperature. While it is a valid method of measuring temperature, it does not use liquid expansion, making it different from the liquid-in-glass thermometer. - **B. Digital thermometer**: - Digital thermometers use electronic sensors to measure temperature and display the reading on a digital screen. They do not rely on the physical expansion of a liquid, so they are fundamentally different from liquid-in-glass thermometers. - **D. Infrared thermometer**: - Infrared thermometers measure temperature by detecting the infrared radiation emitted by an object. They do not involve any liquid or physical contact with the object being measured, which distinguishes them from liquid-in-glass thermometers. ### Common Pitfalls: - **Confusing Types**: Students often confuse different types of thermometers. It's important to remember that liquid-in-glass thermometers specifically rely on liquid expansion, while others use different principles. - **Reading Errors**: When using a liquid-in-glass thermometer, ensure you read the level of the liquid at eye level to avoid parallax errors, which can lead to inaccurate readings. ### Revision Summary: - **Liquid-in-glass thermometers** measure temperature through the expansion of liquids (mercury or alcohol). - They consist of a glass tube with a liquid that expands and contracts with temperature changes. - Other types of thermometers (bimetallic, digital, infrared) use different principles and do not involve liquid expansion. - Always read the thermometer at eye level to avoid parallax errors for accurate measurements.