Thermometers

The instrument used to measure temperature is the thermometer. It can use several physical quantities as a temperature measure, including the volume of a liquid, the length of a bar, the electrical resistance of a wire, etc. Thus, mercury can be used for low temperatures, alcohol for very low temperatures and, on the contrary, a thermocouple or the expansion of a bar for high temperatures can be used.
For this, it was necessary to take a reference, that is, all thermometers must provide the same temperature in a given controlled situation. Thus, in 1954, at the Tenth Weights and Measures Conference, temperature measurements were redefined in terms of a single fixed point. That fixed point
it was chosen from water, that is, a point where ice, liquid water and water vapor coexist in equilibrium: the triple point of water. This point can only be reached for the same pressure (the water vapor pressure is 4.58 mmHg). THE
The temperature of this fixed point (triple point) was established as standard, that is, as 273.16 degrees Kelvin and 0.01 degrees on the Celsius scale. An absolute temperature scale can also be observed. Based on the second law of thermodynamics, a temperature scale can be defined that is independent of the thermometric substance. This absolute scale is usually called the Thermodynamic Temperature Scale.