Mercury Thermometers
In 1714, the mercury-in-glass thermometer was invented in Europe by physicist Daniel Gabriel Fahrenheit. Typically, the mercury is contained in a reservoir that opens to narrow glass tube. The distance that the mercury flows into the tube is dependent on the temperature to which the thermometer is subjected. The space above the mercury may be filled with air or an inert gas, or it may be a partial vacuum.
Thermometers are calibrated by exposing them to a known temperature and then marking the height of the mercury at that temperature. For example, one might calibrate to the freezing temperature of water (0°C or 32°F) by placing the thermometer in a mixture of ice and water. Then the thermometer could be submerged in boiling water to determine the point where the water changes from a liquid to a vapor (100°C or 212°F).
Mercury thermometers are still considered to be some of the most accurate temperature indicators. They can be used for a wide range of temperatures, from -37°C to 356°C (-35°F to 673°F). Modifications such as adding an inert gas above the mercury or replacing the glass with fused quartz can extend the upper temperature limit to as high as 800°C (1,470°F).
Mercury cannot be used below -38.83°C (-37.89°F), the temperature at which it becomes a solid. Moreover, nitrogen in the thermometer can flow down into the column and become trapped if the mercury solidifies, which makes the thermometer unusable. Use a thermometer that contains a mercury-thallium alloy for measuring lower meteorological temperatures — it will remain liquid to -61.1°C (-78.0°F).