What is a thermometer?

We explain what a thermometer is, what it is for and who invented it. In addition, the types of thermometers and how they work.

1. What is a thermometer?

An instrument is called a thermometer whose usefulness is to measure temperature , through various mechanisms and scales. The most common of these mechanisms was dilation, property of certain materials to expand in the face of the presence of heat, common among metals and other substances, such as alcohols .

The invention of the thermometer and its incorporation into everyday life was an important success in the technological development of medicine (clinical thermometer), since it allowed the measurement of the temperature of the human body and accurately measure symptoms such as fever .

The recorded by the thermometers is marked based on a certain temperature scale:

• Celsius (° C) , in honor of the Swedish physicist Andreas Celsius, also known as degrees Celsius .
• Fahrenheit (° F) , proposed by the German physicist Daniel Fahrenheit in 1724, preferably employed in the Anglo-Saxon world.
• Kelvin ( ° K ) . The absolute temperature scale  is used in the International System of Units. It is coincident with the Celsius scale, but 0 has been set at the so-called “absolute zero”, that is, the lowest temperature that exists: -273.15 ° C.
• Réaumur  ( ° R ) . In disuse at present, due to René-Antoine Ferchault de Réaumur, French physicist.

2. Who invented the thermometer?

The thermometer is a long-standing device in human history, whose first version was called a  thermoscope  and invented by the Renaissance scientist Galileo Galilei: it consisted of a glass vessel that culminated in a closed sphere, which had to be submerged upside down in a mixture of alcohol and water , leaving the sphere above. As the liquid warmed, it climbed the tube.

To this invention a numerical scale was added between 1611 and 1613 (attributed to Francesco Sagredo and Santorio Santorio), thus giving rise to the first thermometers and also to thermoscopes, devices that measure the ambient temperature.

3. Thermometer types

There are the following types of thermometer:

• Mercury thermometer . Taking advantage of the enormous expansion capacity of the single liquid metal, these thermometers were manufactured for centuries since its invention in 1714 by the physicist Fahrenheit. They are extremely practical and accurate. They are still used extensively, although their manufacture was banned in certain countries because mercury, once the thermometer’s useful life is exceeded, becomes an environmental pollutant.
• Pyrometers . Used in foundries and factories, where it is required to measure the exact temperature (very high), they operate based on various mechanisms: the collection of infrared radiation, the distribution of thermal radiation (based on color), and even the photoelectric effect.
• Gas thermometer . Under constant pressure and volume, certain gases are used based on their ability to expand when heated. This yields very accurate results and is therefore used to calibrate other thermometers.
• Bimetallic sheet thermometer . It consists of two sheets of metals that have different expansion coefficients, folded so that the one with the highest coefficient is inside. This is how the temperature sensor operates in a thermohyggraph.
• Digital thermometers . They operate based on specialized electronic circuits and sensors, capable of measuring small voltage variations and translating them into digits within one of the temperature scales (or several).
• Clinical thermometers . So called thermometers especially used in medicine, to measure body temperature. They are usually made of glass (mercury) or plastic (digital).

4. How does a thermometer work?

The principle governing the thermometer is simple: the device has a sensitive end, where the sensors (in the case of a digital thermometer) or the dilatable substance (in the case of mercury or alcohol thermometers) are located, and that It must be introduced into the body or the substance whose temperature you want to measure.

After waiting a few minutes, the heat of the body or the substance will bring the mercury or alcohol up to an equivalent point, on the scale recorded in the device, to the degree of heat measured.