The correct option is **C. Mercury thermometer**. ### Detailed Explanation 1. **Understanding Thermometers**: Thermometers are devices used to measure temperature. They operate on different principles depending on the type of thermometer. 2. **Expansion of Liquid**: The key principle behind the mercury thermometer is the thermal expansion of liquids. When a liquid is heated, it expands. This expansion can be measured to determine the temperature. 3. **How a Mercury Thermometer Works**: - **Construction**: A mercury thermometer consists of a glass tube that contains mercury, a metal that expands uniformly when heated. - **Measurement**: As the temperature increases, the mercury expands and rises in the glass tube. The height of the mercury column corresponds to a specific temperature on a calibrated scale (usually Celsius or Fahrenheit). - **Calibration**: The thermometer is calibrated at known temperature points (like the freezing point of water at 0°C and the boiling point at 100°C) to ensure accurate readings. 4. **Why Option C is Correct**: The mercury thermometer directly utilizes the expansion of mercury to measure temperature. This is a straightforward and effective method for measuring temperature in many applications. ### Why the Other Options are Incorrect - **A. Bimetallic thermometer**: - **Explanation**: A bimetallic thermometer uses two different metals that expand at different rates when heated. These metals are bonded together, and as they expand, they bend, moving a pointer on a dial to indicate temperature. - **Why it's weaker**: While it does measure temperature, it does not use the expansion of a liquid, which is the specific criterion in the question. - **B. Thermocouple**: - **Explanation**: A thermocouple consists of two different metals joined at one end. When the junction is heated, it produces a voltage that can be correlated to temperature. - **Why it's weaker**: This method does not involve the expansion of a liquid; instead, it relies on the thermoelectric effect, making it unsuitable for the question. - **D. Infrared thermometer**: - **Explanation**: An infrared thermometer measures the infrared radiation emitted by an object to determine its temperature. - **Why it's weaker**: This type of thermometer does not involve any liquid or its expansion; it measures temperature from a distance based on emitted radiation. ### Common Pitfalls - **Confusing Different Types**: Students often confuse the principles behind different thermometers. It's essential to remember that the question specifically asks for a thermometer that uses liquid expansion. - **Overlooking Calibration**: Understanding how thermometers are calibrated can help clarify why certain types are more accurate than others. ### Revision Summary - **Mercury thermometers measure temperature through the expansion of mercury, a liquid.** - **Bimetallic thermometers use the bending of metals, not liquids.** - **Thermocouples rely on voltage generation from different metals, while infrared thermometers measure emitted radiation.** - **Always remember the specific principles behind each type of thermometer to avoid confusion.**