Temperature > 08 – Thermometric Scales

Using the Zeroth Law of Thermodynamics, many scientists have tried to build devices that would reveal the temperature of a body. But how could the Zeroth Law help them?

Imagine the following, based on a mercury thermometer and an experiment to measure the temperature of water in a container: the system is composed of the water contained in the container, the material from which the thermometer is made, and mercury (indicator material).

After some time, with the mercury in contact with the water, the mercury expands or contracts and indicates the temperature of the water at each instant. In other words, the container is in thermal equilibrium with the water, which is in thermal equilibrium with the material from which the thermometer is made (usually glass). And, since the mercury is in equilibrium with the glass, according to the Zeroth Law, it is also in thermal equilibrium with the water, and thus demonstrates its temperature.

Perhaps the Celsius scale is the only one you are familiar with, since it is the most popular. The Kelvin and Fahrenheit scales are most commonly used in the scientific world.

It would be interesting to make a comparison: note that the melting point differs in the three scales: Celsius (0°C), Fahrenheit (32°F) and Kelvin (273K).

Note that the same occurs with the boiling point: Celsius (100°C), Fahrenheit (212°F) and Kelvin (373K).

How did each of these scales come about? To understand, let's look back at a little history:

Celsius scale: was developed in 1742 by the Swedish astronomer Anders Celsius (1701-1744). He established fixed points on his scale as the melting point of ice (0°C) and the boiling point of water (100°C).

Fahrenheit scale: was created based on studies carried out by Daniel Gabriel Fahrenheit (1686-1736), around 1742. It is the most widely used scale in English-speaking countries. He determined that water turns to ice at a temperature of 32°F and boils at a temperature of 212°F.

Kelvin scale: originated from the principles established by Lord Kelvin (1824-1907), a physicist of Irish origin, who attributed the absolute zero of his scale as being equal to -273°C on the Celsius scale.