Mercury thermometer

Ferdinand II de Medici was first to invent a closed thermometer (immune from effects of pressure), but used water or alcohol which didn't expand evenly with temperature. Fahrenheit made the switch to Mercury to remedy this. The scale is named after him, which starts at 0 because it was the coldest temperature he could create.
 Daniel Gabriel Fahrenheit Germany 1714 Mercury thermometer

Ferdinand II de Medici was first to invent a closed thermometer (immune from effects of pressure), but used water or alcohol which didn't expand evenly with temperature. Fahrenheit made the switch to Mercury to remedy this. The scale is named after him, which starts at 0 because it was the coldest temperature he could create.