Temperature is one of the most fundamental physical quantities. It is surprisingly difficult to measure: A temperature scale requires two well-defined points to establish reference temperatures, and a thermometer must cover the entire temperature range of interest. It was not until the 18th century that thermometers could be sufficiently calibrated.
In this article, we will discuss the working principle of the thermometer, before delving into the physical definition of the three popular temperature scales: Celsius, Fahrenheit, and Kelvin. Finally, we will take a look at the physical extremes: matter near absolute zero, and the hottest objects in the universe.