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What is the Thermometer ?

The thermometer is an instrument used to measure the temperature of the environment as well as that of the human body, which serves as the basis for detecting symptoms such as fever.

Historically the term thermometer appeared, publicly, in 1624 and is derived from two Greek words: θερμός, thermo, which means «hot» and μέτρον, metron, which means «measure». Later, in 1714, Gabriel Fahrenheit invented the well-known mercury thermometer, whose insertion in society produced important changes in relation to the development of medicine through technological instruments.

What is the Thermometer Used for ?

The thermometer is an instrument used to measure the temperature of the environment, body, food, liquid elements and / or gases; based on thermometric properties.

Types of Thermometers

There are several types of thermometers, each designed for a specific purpose. The most common are those for domestic use to measure body temperature, those that measure ambient temperature. They can be found in their traditional glass form, and the digital readout thermometer has become common.

Glass or Liquid Thermometer

The glass thermometer, also called a liquid thermometer, is the most traditionally known and prototypical. It is also called a manual or mercury thermometer, as it traditionally contains this liquid metal and does not require batteries for its operation.

With technological development, the mercury contained in the first glass thermometers has been replaced by other substances, to prevent exposure to its toxicity.

Bimetallic Foil Thermometers

There are bimetallic foil thermometers, which measure temperature using a mechanism that contains two different types of metal. They make it possible to determine the temperature of an object or medium based on its contraction or expansion.

Digital Thermometers

The novel digital thermometers retain a great similarity with those of glass, since they originated in these but substituting their components towards the use of electronic components. In this century they have almost completely replaced the popularity of the classic models, and offer the advantage of quick and easy readings. If anything, the digital thermometer has its own drawbacks compared to glass ones.

Non-Contact Pyrometers or Thermometers

The pyrometer, or non-contact thermometer, is a particular type of thermometer that operates without touching the object whose temperature is to be measured, using the thermal changes detected by its infrared.

Its use has become common in workplaces with many employees or workers, such as at the entrance of business buildings and factories or plants; in order to detect – according to a thermal level – those who, for example, may have temperatures associated with viral conditions that may be evolving asymptomatically.

Gas Thermometers

Gas thermometers are less widely used instruments at home, but they are still highly appreciated in the industrial sector. The base gas of its operation is usually Nitrogen, by means of which it is possible to measure the precision and reliability of other thermal instruments.

Thermal Pair or Thermocouple

The thermal torque thermometer or thermocouple is similar to the resistance thermometer, in the sense that both measure the temperature from an electrical resistance that induces a voltage. The temperature is measured as a function of the action of the measured object on the device.

Clinical Thermometers

This is the name given to a type of thermometer used especially for health purposes in hospitals and clinics.

Parts of a Thermometer

Glass Tube

It refers to the body of the thermometer, which is usually made from neutral borosilicate glass, measuring about a foot long. It is of fine diameter, so it needs to be used with due care, due to its fragility.


It refers to the widening of the tube, which acts as a reservoir for the fluid that contains the thermometer. The bulb is the place where mercury is stored in traditional thermometers, and it can be cylindrical or pear-shaped depending on the type of thermometer.


This part is known as a stem, and it refers to an inner tube that is displayed inside the tube that serves as a glass cover. It is tiny in diameter and has a flare at its lower end. Its design allows mercury to move when temperature changes occur.


It is presented on the outside of the glass tube as a subdivided interval, although they also come in presentations on a plastic plate that is located behind the capillary. The scale usually shows a certain scalar unit of measurement and a range that is usually unique to each thermometer.

Thermometric Substance

It refers to the liquid that each thermometer contains inside, and it is usually colored alcohol (for environments in temperatures below freezing for others) or mercury.

Construction Valve

It appears as a neck or thinning that communicates the stem with the bulb. Its function is to prevent the mercury from returning to the bulb when the heat received by the thermometer is removed.

How Does the Thermometer Work ?

The principle of operation of the thermometer is relatively simple: it has a sensitive end in which sensors are located (in the case of a digital thermometer), or the expandable substance (in the case of a mercury or alcohol thermometer); and that it must be introduced into the medium, body or substance whose temperature is to be known.

After waiting a few seconds or minutes, the heat of the medium, body or substance makes the mercury or alcohol rise. The level reached is a reading equivalent, on the scale registered on the device, to the degree of heat measured.

What Does the Thermometer Measure ? Unit of Measurement

The most common units of measurement in these devices are degrees Celsius (also called centigrade), Fahrenheit and Kelvin. Scientific community often uses degrees Celsius, but nations like the United States, Liberia, and Burma still measure temperature using degrees Fahrenheit. The Kelvin scale is used in the scientific community to record temperatures that require greater precision. Thermometer units of measurement are marked based on a given temperature scale:

Celsius (° C)

In honor of the Swedish physicist Andreas Celsius, also known as degrees Celsius.

Fahrenheit (° F)

Proposed in 1724 by the German physicist Daniel Fahrenheit, and which continues to be used in Anglo-Saxon society.

Kelvin (° K)

It is an absolute temperature scale that is usually used in the International System of Units. It coincides with the Celsius scale, but the value «zero» has been fixed at the so-called «absolute zero». The lowest temperature that exists: -273.15 ° C.

Réaumur (° R)

Little known, no longer in use today, named after the French physicist René-Antoine Ferchault de Réaumur.

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What is Thermometer Used For, Definition, Uses, Types, How Does it Work, What it measures, Unit of measure and More
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