What is a thermometer?

A thermometer is a device that measures temperature or a temperature gradient (how hot or cold an object is).

A thermometer has two important parts:

  1. A temperature sensor (for example the bulb of a glass mercury thermometer or the pyrometric sensor of an infrared thermometer) in which a change occurs with a change in temperature.

  2. A way to convert this change to a digital value (for example, the visible scale that is marked on a mercury thermometer in the glass or the digital readout on an infrared model).

Thermometers are widely used in technology and industry to monitor processes, in meteorology, medicine, and scientific research. 

Some of the principles of the thermometer were known to Greek philosophers two thousand years ago. The Italian physician Santorio Santorio (Sanctorius, 1561-1636) is generally credited with the invention of the first thermometer, but its standardization was completed during the 17th and 18th centuries. In the first decades of the 18th century in the Dutch Republic. Daniel Gabriel Fahrenheit made two revolutionary breakthroughs in the history of thermometry. He invented the glass mercury thermometer (the first widely used, accurate and convenient thermometer) and the Fahrenheit scale (the first standardized temperature scale to be widely used).


An infrared thermometer is a kind of pyrometer.

While an individual thermometer is capable of measuring degrees of heat, the readings of two thermometers cannot be compared unless they meet an agreed scale. Today there is an absolute thermodynamic temperature scale. The most recent official temperature scale is the 1990 International Temperature Scale.

Thermometer with Fahrenheit (° F symbol) and Celsius (° C symbol) units.

The precision of thermometry

In 1714, Dutch scientist and inventor Daniel Gabriel Fahrenheit invented the first reliable thermometer, using mercury instead of mixtures of alcohol and water. In 1724 he proposed a temperature scale which now (slightly adjusted) bears his name. He could do this because he was first making thermometers, using mercury (which has a high coefficient of expansion), and the quality of his output could provide a finer scale and greater reproducibility, leading to his general adoption. In 1742 Anders Celsius (1701–1744) proposed a scale with zero at the boiling point and 100 degrees at the freezing point of water, although the scale that now bears his name reversed them.

The first physician to use thermometer measurements in clinical practice was Herman Boerhaave (1668–1738).

Thermometric materials

There are different types of empirical thermometers based on the properties of materials.

Many empirical thermometers rely on the constitutive relationship between the pressure, volume and temperature of their thermometric material. For example, mercury expands when heated.

If used for its relationship between pressure and volume and temperature, a thermometric material must have three properties:

  1. Its heating and cooling must be fastThat is, when a quantity of heat enters or leaves a body of the material, the material must expand or contract to its final volume or reach its final pressure and must reach its final temperature practically Without delay; part of the heat that enters can be considered to change the volume of the body at constant temperature, and is called the latent heat of expansion at constant temperature; and the rest can be considered to change body temperature at constant volume, and is called specific heat at constant volume. Some materials do not have this property and take some time to distribute the heat between the change in temperature and volume.

  2. Its heating and cooling must be reversible. That is, the material must be able to be heated and cooled indefinitely often by the same increment and decrement of heat, and always return to its original pressure, volume and temperature each time.

  3. Its heating and cooling should be monotonous. That is to say, it has over the entire temperature range for which it must operate a constant pressure or a constant volume.

At temperatures of around 4 ° C, water does not have these properties, so it behaves abnormally in this regard. Water cannot therefore be used as a material for this type of thermometry for temperature ranges close to 4 ° C.

Gases, on the other hand, have all of these properties. Therefore, they are suitable thermometric materials, and that is why they played an important role in the development of thermometry.

Primary and secondary thermometers

A thermometer is called a primary or secondary depending on how the gross physical quantity it measures corresponds to a temperature. For primary thermometers, the measured property of matter is so well known that the temperature can be calculated without any unknown quantity. Examples of these are thermometers based on the equation of state of a gas or on the speed of sound in a gas.

In contrast, "secondary thermometers are the most widely used because of their convenience. In addition, they are often much more sensitive than primary thermometers. For secondary thermometers, knowledge of the property being measured is not sufficient to allow a direct calculation of the temperature They must be calibrated.


Thermometers can be calibrated either by comparing them with other calibrated thermometers or by comparing them to known fixed points on the temperature scale. The best known of these fixed points are the melting and boiling points of pure water. (Note that the boiling point of water varies with pressure, so this must be controlled.)

The easiest way to calibrate a liquid-in-glass or liquid-in-metal thermometer involved three steps:

  1. Immerse the sensing part in a stirred mixture of pure ice and water at atmospheric pressure and mark the indicated point when it has reached thermal equilibrium.

  2. Immerse the sensing part in a steam bath at standard atmospheric pressure and mark the indicated point again.

  3. Divide the distance between these marks into equal portions depending on the temperature scale used.

Resolution, precision and reproducibility

La resolution of a thermometer is simply what fraction of a degree it is possible to take a reading. For high temperature work, it may be possible to measure only to within 10 ° C or more. Clinical thermometers and many electronic thermometers are usually readable at 0,1 ° C. Special instruments can give readings to the thousandth of a degree. However, this temperature display does not mean that the reading is true or accurate, it only means that very small changes can be observed.

La precision of a calibrated thermometer is given to a known and accurate fixed point (i.e. it gives a true reading) at that point. Between fixed calibration points, interpolation is usually done in a linear fashion. This can give significant differences between different types of thermometers at points far from fixed points. For example, the expansion of mercury in a glass thermometer is slightly different from the change in resistance of a platinum resistance thermometer, so these two will disagree slightly.

La reproducibility is important. In other words, does the same thermometer give the same reading for the same temperature? Reproducible temperature measurement means that comparisons are valid in scientific experiments and industrial processes are consistent. So if the same type of thermometer is calibrated in the same way, its readings will be valid even if they are slightly inaccurate from the absolute scale.

An example of a reference thermometer used to check others against industry standards would be a platinum resistance thermometer with a digital display at 0,1 ° C (its accuracy) that has been calibrated at 5 points (−18, 0, 40 , 70, 100 ° C) and the accuracy of which is ± 0,2 ° C.

Properly calibrated, operated and maintained liquid-glass thermometers can achieve a measurement uncertainty of ± 0,01 ° C in the range of 0 to 100 ° C.


Thermometers use a range of physical effects to measure temperature. Temperature sensors are used in a wide variety of scientific and technical applications, especially measurement systems. Temperature systems are primarily electrical or mechanical, sometimes inseparable from the system they control (as in the case of a glass mercury thermometer). Thermometers are used on roads in cold climates to help determine if icing conditions exist. Indoors, thermistors are used in air conditioning systems such as air conditioners, freezers, radiators, refrigerators and water heaters. Galileo thermometers are used to measure indoor air temperature, due to their limited measuring range.

These liquid crystal thermometers (which use thermochromic liquid crystals) are also used to measure water temperature in aquariums.

Fiber Bragg grating temperature sensors are used in nuclear power plants to monitor reactor core temperatures and avoid the possibility of nuclear meltdown.


Nanothermometry is an emerging field of research dealing with the knowledge of temperature at the submicron scale. Conventional thermometers cannot measure the temperature of an object that is smaller than a micrometer, and new methods and materials must be used. Nanothermometry is used in such cases. 


Thermometers used specifically for low temperatures.


Ear thermometers are generally infrared thermometers.

The forehead thermometer is an example of a liquid crystal thermometer.

Rectal and oral thermometers are generally mercury-based, but have since been largely replaced by digital readout NTC thermistors.

Various thermometric techniques have been used throughout history from the Galileo thermometer to thermal imaging. Medical thermometers such as mercury in glass thermometers, infrared thermometers, pill thermometers, and liquid crystal thermometers are used in healthcare settings to determine whether people have a fever or are hypothermic.

Food and food security

Thermometers are important to food safety, where foods at given temperatures can be subject to potentially harmful levels of bacterial growth after several hours, which could lead to foodborne illness. This includes monitoring refrigeration temperatures and maintaining the temperature of foods served under heat lamps or hot water baths. Cooking thermometers are important in determining whether a food is properly cooked. In particular, meat thermometers are used to help cook meat to a safe internal temperature while preventing overcooking. They are usually found using either a bimetallic coil or a thermocouple or thermistor with a digital readout. Candy thermometers are used to help achieve a specific water content in a sugar solution based on its boiling temperature.


Indoor-outdoor thermometer

The heat meter uses a thermometer to measure the heat flow.

Thermostats used bimetallic strips, but digital thermistors have since become popular.

Alcohol thermometers, infrared thermometers, recording thermometers and thermistors are used in meteorology and climatology at various levels of the atmosphere and oceans. Aircraft use thermometers and hygrometers to determine if atmospheric icing conditions exist along their flight path. These measurements are used to initialize the weather forecast models. Thermometers are used on roads in cold climates to help determine if icing conditions exist and indoors in air conditioning systems.

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