In order to measure body temperature, a device called a thermometer has been developed.
The most common thermometer is mercury, which consists of a graduated glass with a thin-walled bulb that is attached to a very thin tube called a capillary tube.
As the temperature of the thermometer rises, the mercury molecules increase their agitation causing it to expand, filling the capillary tube. For each height reached by mercury a temperature is associated.
The scale of each thermometer corresponds to this reached height value.
It is the scale used in Brazil and most countries, made official in 1742 by the Swedish astronomer and physicist Anders Celsius (1701-1744). This scale has as reference points the freezing temperature of water under normal pressure (0 ° C) and the boiling temperature of water under normal pressure (100 ° C).
Another widely used scale, mainly in English-speaking countries, created in 1708 by German physicist Daniel Gabriel Fahrenheit (1686-1736), having as reference the temperature of a mixture of ice and ammonium chloride (0 ° F) and the temperature of human body (100 ° F).
Compared to the Celsius scale:
0 ° C = 32 ° F
100 ° C = 212 ° F
Also known as absolute scale, it was verified by the English physicist William Thompson (1824-1907), also known as Lord Kelvin. This scale is based on the temperature of the lowest stirring state of any molecule (0 K) and is calculated from the Celsius scale.
By convention, you do not use "degree" for this scale, that is 0 K, you read zero kelvin and not zero degree kelvin. Compared to the Celsius scale:
-273 ° C = 0 K
0 ° C = 273 K
100 ° C = 373 K