The correct option is **C. Liquid-in-glass thermometer**. ### Explanation of the Correct Answer A liquid-in-glass thermometer is a device that measures temperature by utilizing the principle of thermal expansion of a liquid, typically mercury or colored alcohol, contained within a glass tube. Here’s how it works: 1. **Thermal Expansion**: When the temperature increases, the liquid inside the glass tube expands. Conversely, when the temperature decreases, the liquid contracts. This property of liquids to expand and contract with temperature changes is fundamental to the operation of this type of thermometer. 2. **Glass Tube**: The liquid is housed in a narrow glass tube that is calibrated with a scale (usually in degrees Celsius or Fahrenheit). As the liquid expands, it rises in the tube, and the level of the liquid can be read against the scale to determine the temperature. 3. **Calibration**: The thermometer is calibrated at specific points (like the freezing point of water at 0°C and the boiling point at 100°C) to ensure accurate readings. The scale is marked accordingly, allowing users to easily read the temperature. ### Why Other Options Are Incorrect - **A. Digital thermometer**: - Digital thermometers measure temperature using electronic sensors, such as thermocouples or thermistors, and display the temperature digitally. They do not rely on the physical expansion of a liquid, which is the defining characteristic of a liquid-in-glass thermometer. - **B. Bimetallic thermometer**: - A bimetallic thermometer consists of 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 also measures temperature, it does not use liquid expansion, making it different from a liquid-in-glass thermometer. - **D. Infrared thermometer**: - Infrared thermometers measure temperature by detecting the infrared radiation emitted by an object. They do not involve any physical contact with the object being measured and do not use liquid expansion. This method is entirely different from the liquid-in-glass principle. ### Common Pitfalls - **Confusing Types**: Students often confuse different types of thermometers. It’s important to remember that the defining feature of a liquid-in-glass thermometer is the use of liquid expansion, while other types use different principles. - **Reading Scales**: Misreading the scale on a liquid-in-glass thermometer can lead to incorrect temperature readings. Always ensure you are reading at eye level to avoid parallax errors. ### Summary for Revision - A liquid-in-glass thermometer measures temperature through the expansion of a liquid (mercury or alcohol) in a glass tube. - The liquid expands with heat and contracts with cold, allowing for accurate temperature readings against a calibrated scale. - Other thermometer types (digital, bimetallic, infrared) use different principles and do not rely on liquid expansion. - Always read the thermometer at eye level to avoid errors in measurement.