Thermometers and thermometric scales

Nowadays, if we get a fever, we can measure body temperature with the most varied types of thermometers, like the traditional ones, with capillary tubes containing Mercury, and digital thermometers. We know that it has not always been this easy, as, some time ago, to check if a person had fever, it was necessary for another person to touch his forehead to take his temperature.

Historically, it is known that the first thermometer was formed by a rounded part of glass, called a bulb, and a thin glass neck. This type of thermometer was invented by the great physicist, mathematician and astronomer Galileo Galilei. Galileo heated the bulb, removing some of the air that was inside, so that he could capsize the tube in the water. After dipping the tube into the container with water and dye, the bulb temperature returned to its normal value, causing the water to rise through the tube to a certain height.

In this way, he was able to make comparisons between the most varied objects that were placed in contact with the bulb of his thermometer, as he observed that the height of the water column depended on the temperature of the object, that is, the higher the temperature, the higher the water column would be. Water. Therefore, Galileo performed temperature measurements indirectly by comparison.

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Based on the thermometer built by Galileo, several other scientists were also dedicated to the construction of this equipment. Although liquid-based thermometers were invented many years ago (around 400 years), the common thermometer, like the one in the picture above, with mercury inside a glass tube, has only been widely used in the last eighty years and is expected to fall into disuse because of the danger mercury poses to our health.

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thermometric scales

By varying the temperature, the level of thermal agitation also varies. However, this variation is not always noticeable to the naked eye. If the measurement of thermal agitation is not possible, the thermometer, constructed of a material whose thermometric magnitude varies with temperature and is perceptible to the naked eye.

In order to graduate a thermometric scale, become reference points of thermal states well defined and easily obtainable, called fixed points, obtained under normal pressure. The first fixed point is the ice point (melting point), and the second is the steam point (boiling water).

Below, we have the main thermometric scales:

Celsius scale

On the Celsius scale, the ice point is 0 and vapor point is 100. On this scale, the interval between fixed points is divided into 100 equal parts, and each division corresponds to 1 degree.

Fahrenheit scale

On the Fahrenheit scale, the ice point is 32 and vapor point is 212. On this scale, the interval between the two fixed points is divided into 180 equal parts, and each division corresponds to 1 degree Fahrenheit (1°F).

Kelvin scale

On the Kelvin scale, the ice point is 273 and vapor point is 373. On this scale, the interval between the two points is divided into 100 equal parts, and each division corresponds to 1 kelvin (1 k).

Thus, we define the correspondence between these scales as follows:

0°C = 32°F = 273 K
100°C = 212°F = 373 K


By Joab Silas
Graduated in Physics

Would you like to reference this text in a school or academic work? Look:

JUNIOR, Joab Silas da Silva. "Thermometers and thermometric scales"; Brazil School. Available in: https://brasilescola.uol.com.br/fisica/escalas-termometricas.htm. Accessed on June 27, 2021.

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