Which of the following elements is used in thermometers?

Mercury

The correct answer is option 4. Mercury.

Mercury is used in thermometers due to its unique physical properties. Here's an explanation of why mercury is a suitable choice for this application:

1. High Thermal Expansion:  Mercury has a high coefficient of thermal expansion, meaning it expands and contracts significantly with changes in temperature. This property allows the mercury column inside the thermometer to rise or fall in response to temperature variations.

2. Wide Range of Measurement: Mercury remains liquid at a wide range of temperatures, making it suitable for measuring temperatures both below and above room temperature. The liquid state of mercury ensures that it can fill the entire length of the thermometer scale.

3. Visibility: Mercury is highly visible, forming a distinct and easily observable silver or metallic line in the thermometer tube. This visibility makes it simple for users to read the temperature indicated on the scale.

4. Uniform Expansion: Mercury expands uniformly, providing consistent and reliable measurements. This characteristic is essential for accurate temperature readings.

Despite these advantages, it's important to note that mercury has environmental and health concerns associated with its use. Due to the toxicity of mercury, especially in vapor form, there has been a shift toward using alternative, mercury-free thermometers in many applications. Digital thermometers and alcohol-based or other non-mercury liquid-filled thermometers are often preferred in modern settings to mitigate the risks associated with mercury exposure.