**Correct Option: C. Mercury thermometer** ### Explanation of the Correct Answer A mercury thermometer operates on the principle of thermal expansion of liquids to measure temperature. Here’s how it works: 1. **Principle of Thermal Expansion**: When a liquid is heated, it expands. This means that as the temperature increases, the liquid occupies a larger volume. In a mercury thermometer, mercury is the liquid used because it has a high coefficient of thermal expansion, meaning it expands significantly with temperature changes. 2. **Construction of the Mercury Thermometer**: A typical mercury thermometer consists of a glass tube that is sealed at one end and contains mercury. The other end is open to the atmosphere, allowing the mercury to expand and contract freely. The tube is marked with a scale (usually in degrees Celsius or Fahrenheit) that indicates the temperature based on the height of the mercury column. 3. **Measurement Process**: When the thermometer is placed in a substance (like water or air), the mercury inside the tube absorbs heat from the substance. As the temperature of the substance rises, the mercury expands and moves up the tube. The height of the mercury column corresponds to a specific temperature on the scale, allowing for an accurate reading. ### Why Other Options Are Incorrect - **A. Bimetallic thermometer**: This type of thermometer uses two different metals that expand at different rates when heated. These metals are bonded together and form a coil or strip. As the temperature changes, the differing expansion rates cause the bimetallic strip to bend, which moves a pointer on a dial to indicate the temperature. While it also measures temperature, it does not rely on the thermal expansion of a liquid. - **B. Infrared thermometer**: Infrared thermometers measure temperature by detecting the infrared radiation emitted by an object. They do not use any liquid or thermal expansion principle. Instead, they convert the detected infrared radiation into a temperature reading. This method is non-contact and is used for measuring the temperature of objects from a distance. - **D. Thermocouple**: A thermocouple consists of two different metals joined at one end. When the junction is heated or cooled, it produces a voltage that can be correlated to temperature. This method relies on the thermoelectric effect (Seebeck effect) rather than the thermal expansion of a liquid. ### Summary of Key Points - **Mercury thermometers** measure temperature based on the thermal expansion of mercury, a liquid that expands significantly with temperature changes. - **Bimetallic thermometers** use the differential expansion of metals, not liquids. - **Infrared thermometers** detect radiation and do not involve thermal expansion. - **Thermocouples** rely on voltage generation from temperature differences between two metals, not on liquid expansion. ### Revision Summary - Mercury thermometers measure temperature through the thermal expansion of mercury. - They consist of a glass tube with a scale and a sealed end containing mercury. - Other types of thermometers (bimetallic, infrared, thermocouple) use different principles and do not rely on liquid expansion. - Understanding the working principles of different thermometers is crucial for selecting the appropriate one for specific applications.