Thermometers And Fixed Points

A thermometer is an instrument for measuring the temperature. The Zeroth law gives the foundation for temperature measurement: The thermometer plays the role of S₃; it gives a quantitative measure of the disparity in temperature. For this reason, it is possible to determine two standard systems whose circumstances are simply reproducible. One of them is S_ice, a system having a mixture of ice and water at a pressure of one atmosphere; the second is S_steam having water boiling at a pressure of one atmosphere. Let use a mercury-in-glass thermometer to measure the temperature. The location of the meniscus of the mercury column, if it has achieved thermal equilibrium with S_ice is marked and selected as the ice point; the position when thermal equilibrium is attained with S_steam is also marked and known as the steam point. The length of glass tube among these two marks, that are generally called the fixed points, is then splitted into a number of equivalent intervals.