Developed during the 16th and 17th centuries, a thermometer (from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 θερμός (thermos) meaning "warm" and meter, "to measure") is a device that measures temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 or temperature gradient
Temperature gradient
A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location. The temperature gradient is a dimensional quantity expressed in units of degrees per unit length...

 using a variety of different principles. A thermometer has two important elements: the temperature sensor (e.g. the bulb on a mercury thermometer
Mercury-in-glass thermometer
A mercury-in-glass thermometer, also known as a mercury thermometer, was invented by German physicist Daniel Gabriel Fahrenheit in 1724 and is a thermometer consisting of mercury in a glass tube. Calibrated marks on the tube allow the temperature to be read by the length of the mercury within the...

) in which some physical change occurs with temperature, plus some means of converting this physical change into a numerical value (e.g.
Developed during the 16th and 17th centuries, a thermometer (from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 θερμός (thermos) meaning "warm" and meter, "to measure") is a device that measures temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 or temperature gradient
Temperature gradient
A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location. The temperature gradient is a dimensional quantity expressed in units of degrees per unit length...

 using a variety of different principles. A thermometer has two important elements: the temperature sensor (e.g. the bulb on a mercury thermometer
Mercury-in-glass thermometer
A mercury-in-glass thermometer, also known as a mercury thermometer, was invented by German physicist Daniel Gabriel Fahrenheit in 1724 and is a thermometer consisting of mercury in a glass tube. Calibrated marks on the tube allow the temperature to be read by the length of the mercury within the...

) in which some physical change occurs with temperature, plus some means of converting this physical change into a numerical value (e.g. the scale on a mercury thermometer).

There are many types of thermometer and many uses for thermometers, as detailed below in sections of this article.


While an individual thermometer is able to measure degrees of hotness, the readings on two thermometers cannot be compared unless they conform to an agreed scale. There is today an absolute thermodynamic temperature
Thermodynamic temperature
Thermodynamic temperature is the absolute measure of temperature and is one of the principal parameters of thermodynamics. Thermodynamic temperature is an "absolute" scale because it is the measure of the fundamental property underlying temperature: its null or zero point, absolute zero, is the...

 scale. Internationally agreed temperature scales are designed to approximate this closely, based on fixed points and interpolating thermometers. The most recent official temperature scale is the International Temperature Scale of 1990
International Temperature Scale of 1990
The International Temperature Scale of 1990 is an equipment calibration standard for making measurements on the Kelvin and Celsius temperature scales. ITS–90 is an approximation of the thermodynamic temperature scale that facilitates the comparability and compatibility of temperature measurements...

. It extends from 0.65 K (-272.5 °C; -458.5 °F) to approximately 1358 K (1,084.9 °C; 1,984.7 °F).


Various authors have credited the invention of the thermometer to Cornelis Drebbel, Robert Fludd
Robert Fludd
Robert Fludd, also known as Robertus de Fluctibus was a prominent English Paracelsian physician, astrologer, mathematician, cosmologist, Qabalist, Rosicrucian apologist...

, Galileo Galilei
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

 or Santorio Santorio
Santorio Santorio , also called Santorio Santorii, Sanctorius of Padua, and various combinations of these names, was an Italian physiologist, physician, and professor. From 1611 to 1624 he was a professor at Padua where he performed experiments in temperature, respiration and weight...

. The thermometer was not a single invention, however, but a development
Timeline of temperature and pressure measurement technology
Timeline of temperature and pressure measurement technology A history of temperature measurement and pressure measurement technology.-1500s:* 1592-1593 — Galileo Galilei builds a device showing variation of hotness known as the thermoscope using the contraction of air to draw water up a...


Philo of Byzantium
Philo of Byzantium
Philo of Byzantium , also known as Philo Mechanicus, was a Greek engineer and writer on mechanics, who lived during the latter half of the 3rd century BC...

 and Hero of Alexandria
Hero of Alexandria
Hero of Alexandria was an ancient Greek mathematician and engineerEnc. Britannica 2007, "Heron of Alexandria" who was active in his native city of Alexandria, Roman Egypt...

 knew of the principle that certain substances, notably air, expand and contract and described a demonstration in which a closed tube partially filled with air had its end in a container of water. The expansion and contraction of the air caused the position of the water/air interface to move along the tube.

Such a mechanism was later used to show the hotness and coldness of the air with a tube in which the water level is controlled by the expansion and contraction of the air. These devices were developed by several European scientists in the 16th and 17th centuries, notably Galileo Galilei. As a result, devices were shown to produce this effect reliably, and the term thermoscope was adopted because it reflected the changes in sensible heat
Sensible heat
Sensible heat is the energy exchanged by a thermodynamic system that has as its sole effect a change of temperature.The term is used in contrast to a latent heat, which is the amount of energy exchanged that is hidden, meaning it cannot be observed as a change of temperature...

 (the concept of temperature was yet to arise). The difference between a thermoscope
Galileo thermometer
A Galileo thermometer , named after Italian physicist Galileo Galilei, is a thermometer made of a sealed glass cylinder containing a clear liquid and a series of objects whose densities are such that they rise or fall as the temperature changes...

 and a thermometer is that the latter has a scale. Though Galileo is often said to be the inventor of the thermometer, what he produced were thermoscopes.

Galileo Galilei also discovered that objects (glass spheres filled with aqueous
Aqueous solution
An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...

 alcohol) of slightly different densities would rise and fall, which is nowadays the principle of the Galileo thermometer (shown).

Today such thermometers are calibrated to a temperature scale
Scale of temperature
Scale of temperature is a way to measure temperature quantitatively.-Formal description:According to the zeroth law of thermodynamics, being in thermal equilibrium is an equivalence relation. Thus all thermal systems may be divided into a quotient set by this equivalence relation, denoted below as M...


The first clear diagram of a thermoscope was published in 1617 by Giuseppe Biancani
Giuseppe Biancani
Giuseppe Biancani was an Italian Jesuit astronomer, mathematician, and selenographer, after whom the crater Blancanus on the Moon is named...

: the first showing a scale and thus constituting a thermometer was by Robert Fludd in 1638. This was a vertical tube, with a bulb at the top and the end immersed in water. The water level in the tube is controlled by the expansion and contraction of the air, so it is what we would now call an air thermometer.

The first person to put a scale on a thermoscope is variously said to be Francesco Sagredo
Giovanni Francesco Sagredo
Giovanni Francesco Sagredo was a Venetian mathematician and close friend of Galileo, who wrote:Many years ago I was often to be found in the marvelous city of Venice, in discussions with Signore Giovanni Francesco Sagredo, a man of noble extraction and trenchant wit. He was also a friend and...

 or Santorio Santorio in about 1611 to 1613.

The word thermometer (in its French form) first appeared in 1624 in La Récréation Mathématique by J. Leurechon, who describes one with a scale of 8 degrees.

The above instruments suffered from the disadvantage that they were also barometer
A barometer is a scientific instrument used in meteorology to measure atmospheric pressure. Pressure tendency can forecast short term changes in the weather...

s, i.e. sensitive to air pressure. In about 1654 Ferdinando II de' Medici, Grand Duke of Tuscany
Ferdinando II de' Medici, Grand Duke of Tuscany
Ferdinando II de' Medici was grand duke of Tuscany from 1621 to 1670. He was the eldest child of Cosimo II de' Medici and Maria Maddalena of Austria. His 49 year rule was punctuated by the terminations of the remaining operations of the Medici Bank, and the beginning of Tuscany's long economic...

, made sealed tubes part filled with alcohol, with a bulb and stem, the first modern-style thermometer, depending on the expansion of a liquid, and independent of air pressure. Many other scientists experimented with various liquids and designs of thermometer.
However, each inventor and each thermometer was unique—there was no standard scale. In 1665 Christiaan Huygens suggested using the melting
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...

 and boiling point
Boiling point
The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....

s of water as standards, and in 1694 Carlo Renaldini proposed using them as fixed points on a universal scale. In 1701 Isaac Newton
Isaac Newton
Sir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...

 proposed a scale of 12 degrees between the melting point of ice and body temperature
Normal human body temperature
Normal human body temperature, also known as normothermia or euthermia, is a concept that depends upon the place in the body at which the measurement is made, and the time of day and level of activity of the person...

. Finally in 1724 Daniel Gabriel Fahrenheit produced a temperature scale which now (slightly adjusted) bears his name
Fahrenheit is the temperature scale proposed in 1724 by, and named after, the German physicist Daniel Gabriel Fahrenheit . Within this scale, the freezing of water into ice is defined at 32 degrees, while the boiling point of water is defined to be 212 degrees...

. He could do this because he manufactured thermometers, using mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

 (which has a high coefficient of expansion
Thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...

) for the first time and the quality of his production could provide a finer scale and greater reproducibility, leading to its general adoption. In 1742 Anders Celsius
Anders Celsius
Anders Celsius was a Swedish astronomer. He was professor of astronomy at Uppsala University from 1730 to 1744, but traveled from 1732 to 1735 visiting notable observatories in Germany, Italy and France. He founded the Uppsala Astronomical Observatory in 1741, and in 1742 he proposed the Celsius...

 proposed a scale with zero at the boiling point and 100 degrees at the melting point of water, though the scale which now bears his name
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...

 has them the other way around.

In 1866 Sir Thomas Clifford Allbutt invented a clinical thermometer
Medical thermometer
Medical thermometers are used for measuring human body temperature, with the tip of the thermometer being inserted either into the mouth under the tongue , under the armpit , or into the rectum via the anus .-Liquid-filled:The traditional thermometer is a glass tube with a bulb at one end...

 that produced a body temperature reading in five minutes as opposed to twenty. In 1999 Dr. Francesco Pompei
Francesco Pompei
Francesco PompeiFrancesco "Frank" Pompei is the founder and chief executive officer for Exergen Corporation. Pompei earned Bachelor of Science and Master of Science mechanical engineering degrees from the Massachusetts Institute of Technology. He also earned Master of Science and Doctor of...

 of the Exergen Corporation
Exergen Corporation
Exergen Corporation, is designer and manufacturer of infrared scanners, thermometers, and sensors headquartered in Watertown, Massachusetts. Exergen products are used in applications including, among others, medical, automotive, food processing, agriculture and textile...

 introduced the world's first temporal artery thermometer, a non-invasive temperature sensor which scans the forehead in about 2 seconds and provides a medically accurate body temperature.

Old thermometers were all non-registering thermometers. That is, the thermometer did not hold the temperature after it was moved to a place with a different temperature. Determining the temperature of a pot of hot liquid required the user to leave the thermometer in the hot liquid until after reading it. If the non-registering thermometer was removed from the hot liquid, then the temperature indicated on the thermometer would immediately begin changing to reflect the temperature of its new conditions (in this case, the air temperature). Registering thermometers are designed to hold the temperature indefinitely, so that the thermometer can be removed and read at a later time or in a more convenient place. The first registering thermometer was designed and built by James Six
James Six
James Six was a British scientist born in Canterbury. He is noted for his invention, in 1780, of Six's thermometer, commonly known as the Maximum minimum thermometer...

 in 1782, and the design, known as Six's thermometer
Six's thermometer
Six's thermometer is a thermometer which can measure the maximum and minimum temperatures reached over a period of time, usually during a day. It is commonly used wherever a simple way is needed to measure the extremes of temperature at a location, for instance in meteorology and horticulture.It is...

 is still in wide use today. Mechanical registering thermometers hold either the highest or lowest temperature recorded, until manually re-set, e.g., by shaking down a mercury-in-glass thermometer, or until an even more extreme temperature is experienced. Electronic registering thermometers may be designed to remember the highest or lowest temperature, or to remember whatever temperature was present at a specified point in time.

Thermometers increasingly use electronic means to provide a digital display or input to a computer.

Physical principles of thermometry

Thermometers may be described as empirical or absolute. Absolute thermometers are calibrated numerically by the thermodynamic absolute temperature scale Empirical thermometers are not in general necessarily in exact agreement with absolute thermometers as to their numerical scale readings, but to qualify as thermometers at all they must agree with absolute thermometers and with each other in the following way: given any two bodies isolated in their separate respective thermodynamic equilibrium states, all thermometers agree as to which of the two has the higher temperature, or that the two have equal temperatures. For any two empirical thermometers, this does not require that the relation between their numerical scale readings be linear, but it does require that relation to be strictly monotonic
Monotonic function
In mathematics, a monotonic function is a function that preserves the given order. This concept first arose in calculus, and was later generalized to the more abstract setting of order theory....

. This is a fundamental character of temperature and thermometers.

As it is customarily stated in textbooks, taken alone, the so-called 'zeroth law of thermodynamics
Zeroth law of thermodynamics
The zeroth law of thermodynamics is a generalization principle of thermal equilibrium among bodies, or thermodynamic systems, in contact.The zeroth law states that if two systems are in thermal equilibrium with a third system, they are also in thermal equilibrium with each other.Systems are said to...

' fails to deliver this information, but the statement of the zeroth law of thermodynamics by James Serrin
James Serrin
James Serrin is a mathematician known for his contributions to continuum mechanics, nonlinear analysis, and partial differential equations.Serrin was born in Chicago on November 1, 1926. He received his doctorate from Indiana University in 1951...

 in 1977, though rather mathematically abstract, is more informative for thermometry: "Zeroth Law - There exists a topological line which serves as a coordinate manifold of material behaviour. The points of the manifold are called 'hotness levels', and is called the 'universal hotness manifold'." To this information there needs to be added a sense of greater hotness; this sense can be had, independently of calorimetry
Calorimetry is the science of measuring the heat of chemical reactions or physical changes. Calorimetry is performed with a calorimeter. The word calorimetry is derived from the Latin word calor, meaning heat...

, of thermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...

, and of properties of particular materials, from Wien's displacement law of thermal radiation
Thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....

: the temperature of a bath of thermal radiation is proportional
Proportionality (mathematics)
In mathematics, two variable quantities are proportional if one of them is always the product of the other and a constant quantity, called the coefficient of proportionality or proportionality constant. In other words, are proportional if the ratio \tfrac yx is constant. We also say that one...

, by a universal constant, to the frequency of the maximum of its frequency spectrum; this frequency is always positive, but can have values that tend to zero
Third law of thermodynamics
The third law of thermodynamics is a statistical law of nature regarding entropy:For other materials, the residual entropy is not necessarily zero, although it is always zero for a perfect crystal in which there is only one possible ground state.-History:...


There are several principles on which empirical thermometers are built, as listed in the section of this article entitled 'Primary and secondary thermometers'. Several such principles are essentially based on the constitutive relation between the state of a suitably selected particular material and its temperature. Only some materials are suitable for this purpose, and they may be considered as 'thermometric materials'. Radiometric thermometry, in contrast, can be only very slightly dependent on the constitutive relations of materials. In a sense then, radiometric thermometry might be thought of as 'universal'. This is because it rests mainly on a universality character of thermodynamic equilibrium, that it has the universal property of producing blackbody
Black body
A black body is an idealized physical body that absorbs all incident electromagnetic radiation. Because of this perfect absorptivity at all wavelengths, a black body is also the best possible emitter of thermal radiation, which it radiates incandescently in a characteristic, continuous spectrum...


Thermometric materials

There are various kinds of empirical thermometer based on material properties.

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

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

(1) its heating and cooling must be rapid. That is to say, 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 with practically no delay; some 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 of it can be considered to change the temperature of the body at constant volume, and is called the specific heat at constant volume. Some materials do not have this property, and take some time to distribute the heat between temperature and volume change.

(2) its heating and cooling must be reversible. That is to say, the material must be able to be heated and cooled indefinitely often by the same increment and decrement of heat, and still return to its original pressure and volume and temperature every time. Some plastics do not have this property;

(3) its heating and cooling must be monotonic. That is to say, throughout the range of temperatures for which it is intended to work, (a) at a given fixed pressure, either (α) the volume increases when the temperature increases, or else (β) the volume decreases when the temperature increases; not (α) for some temperatures and (β) for others; or (b) at a given fixed volume, either (α) the pressure increases when the temperature increases, or else (β) the pressure decreases when the temperature increases; not (α) for some temperatures and (β) for others.

At temperatures around about 4 °C, water does not have the property (3), and is said to behave anomalously in this respect; thus water cannot be used as a material for this kind of thermometry for temperature ranges about 4 °C.

Gases, on the other hand, all have the properties (1), (2), and (3)(a)(α) and (3)(b)(α). Consequently, they are suitable thermometric materials, and that is why they were important in the development of thermometry.

Constant volume thermometry

According to Preston (1894/1904), Regnault
Henri Victor Regnault
Henri Victor Regnault was a French chemist and physicist best known for his careful measurements of the thermal properties of gases. He was an early thermodynamicist and was mentor to William Thomson in the late 1840s....

 found constant pressure air thermometers unsatisfactory, because they needed troublesome corrections. He therefore built a constant volume air thermometer. Constant volume thermometers do not provide a way to avoid the problem of anomalous behaviour like that of water about 4 °C.

Radiometric thermometry

Planck's law very accurately quantitatively describes the power spectral density of electromagnetic radiation, inside a rigid walled cavity in a body made of material that is completely opaque and poorly reflective, when it has reached thermodynamic equilibrium, as a function of absolute thermodynamic temperature alone. A small enough hole in the wall of the cavity emits near enough blackbody radiation of which the spectral radiance can be precisely measured. The walls of the cavity, provided they are completely opaque and poorly reflective, can be of any material indifferently. This provides a well-reproducible absolute thermometer over a very wide range of temperatures, able to measure the absolute temperature of a body inside the cavity.

Primary and secondary thermometers

Thermometers can be divided into two separate groups according to the level of knowledge about the physical basis of the underlying thermodynamic laws and quantities. For primary thermometers the measured property of matter is known so well that temperature can be calculated without any unknown quantities. Examples of these are thermometers based on the equation of state of a gas, on the velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...

 of sound in a gas, on the thermal noise (see Johnson–Nyquist noise
Johnson–Nyquist noise
Johnson–Nyquist noise is the electronic noise generated by the thermal agitation of the charge carriers inside an electrical conductor at equilibrium, which happens regardless of any applied voltage...

) voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

 or current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...

 of an electrical resistor, on blackbody radiation, and on the angular anisotropy
Anisotropy is the property of being directionally dependent, as opposed to isotropy, which implies identical properties in all directions. It can be defined as a difference, when measured along different axes, in a material's physical or mechanical properties An example of anisotropy is the light...

 of gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...

 emission of certain radioactive
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...

 in a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

. Primary thermometers are relatively complex.

Secondary thermometers are most widely used because of their convenience. Also, they are often much more sensitive than primary ones. For secondary thermometers knowledge of the measured property is not sufficient to allow direct calculation of temperature. They have to be calibrated against a primary thermometer at least at one temperature or at a number of fixed temperatures. Such fixed points, for example, triple point
Triple point
In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases of that substance coexist in thermodynamic equilibrium...

s and superconducting
Superconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...

 transitions, occur reproducibly at the same temperature.


Thermometers can be calibrated either by comparing them with other calibrated thermometers or by checking them against 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 traditional method of putting a scale on a liquid-in-glass or liquid-in-metal thermometer was in three stages:
  1. Immerse the sensing portion in a stirred mixture of pure ice and water at 1 Standard atmosphere
    Atmosphere (unit)
    The standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of pressure. For practical purposes it has been replaced by the bar which is 105 Pa...

     (101.325 kPa
    Pascal (unit)
    The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

    ; 760.0 mmHg) and mark the point indicated when it had come to thermal equilibrium.
  2. Immerse the sensing portion in a steam bath at 1 Standard atmosphere (101.325 kPa; 760.0 mmHg) and again mark the point indicated.
  3. Divide the distance between these marks into equal portions according to the temperature scale being used.

Other fixed points were used in the past are the body temperature (of a healthy adult male) which was originally used by Fahrenheit as his upper fixed point (96 °F (35.6 °C) to be a number divisible by 12) and the lowest temperature given by a mixture of salt and ice, which was originally the definition of 0 °F (-17.8 °C). (This is an example of a Frigorific mixture
Frigorific mixture
A frigorific mixture is a mixture of two or more chemicals that reaches an equilibrium temperature that is independent of the temperature of any of its component chemicals before they are mixed...

). As body temperature varies, the Fahrenheit scale was later changed to use an upper fixed point of boiling water at 212 °F (100 °C).

These have now been replaced by the defining points in the International Temperature Scale of 1990, though in practice the melting point of water is more commonly used than its triple point, the latter being more difficult to manage and thus restricted to critical standard measurement. Nowadays manufacturers will often use a thermostat
A thermostat is the component of a control system which regulates the temperature of a system so that the system's temperature is maintained near a desired setpoint temperature. The thermostat does this by switching heating or cooling devices on or off, or regulating the flow of a heat transfer...

 bath or solid block where the temperature is held constant relative to a calibrated thermometer. Other thermometers to be calibrated are put into the same bath or block and allowed to come to equilibrium, then the scale marked, or any deviation from the instrument scale recorded. For many modern devices calibration will be stating some value to be used in processing an electronic signal to convert it to a temperature.

Precision, accuracy, and reproducibility

The precision or resolution of a thermometer is simply to what fraction of a degree it is possible to make a reading. For high temperature work it may only be possible to measure to the nearest 10 °C or more. Clinical thermometers and many electronic thermometers are usually readable to 0.1 °C. Special instruments can give readings to one thousandth of a degree. However, this precision does not mean the reading is true or accurate.

Thermometers which are calibrated to known fixed points (e.g. 0 and 100 °C) will be accurate (i.e. will give a true reading) at those points. Most thermometers are originally calibrated to a constant-volume gas thermometer
Gas thermometer
A gas thermometer measures temperature by the variation in volume or pressure of a gas. One common apparatus is a constant volume thermometer. It consists of a bulb connected by a capillary tube to a manometer. The bulb is filled with a gas such that the volume of the gas in the bulb remains...

. In between a process of interpolation
In the mathematical field of numerical analysis, interpolation is a method of constructing new data points within the range of a discrete set of known data points....

 is used, generally a linear one. This may give significant differences between different types of thermometer at points far away from the fixed points. For example the expansion of mercury in a glass thermometer is slightly different from the change in resistance of a platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...

 resistance of the thermometer, so these will disagree slightly at around 50 °C. There may be other causes due to imperfections in the instrument, e.g. in a liquid-in-glass thermometer if the capillary tube
Capillary action
Capillary action, or capilarity, is the ability of a liquid to flow against gravity where liquid spontanously rise in a narrow space such as between the hair of a paint-brush, in a thin tube, or in porous material such as paper or in some non-porous material such as liquified carbon fiber, or in a...

 varies in diameter.

For many purposes reproducibility is important. That is, does the same thermometer give the same reading for the same temperature (or do replacement or multiple thermometers give the same reading)? Reproducible temperature measurement
Temperature measurement
Attempts of standardized temperature measurement have been reported as early as 170 AD by Claudius Galenus. The modern scientific field has its origins in the works by Florentine scientists in the 17th century. Early devices to measure temperature were called thermoscopes. The first sealed...

 means that comparisons are valid in scientific experiments and industrial processes are consistent. Thus if the same type of thermometer is calibrated in the same way its readings will be valid even if it is slightly inaccurate compared to the absolute scale.

An example of a reference thermometer used to check others to industrial standards would be a platinum resistance thermometer
Resistance thermometer
Resistance thermometers, also called resistance temperature detectors or resistive thermal devices , are sensors used to measure temperature by correlating the resistance of the RTD element with temperature. Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass...

 with a digital display to 0.1 °C (its precision) which has been calibrated at 5 points against national standards (−18, 0, 40, 70, 100 °C) and which is certified to an accuracy of ±0.2 °C.

According to British Standards
British Standards
British Standards are the standards produced by BSI Group which is incorporated under a Royal Charter...

, correctly calibrated, used and maintained liquid-in-glass thermometers can achieve a measurement uncertainty of ±0.01 °C in the range 0 to 100 °C, and a larger uncertainty outside this range: ±0.05 °C up to 200 or down to −40 °C, ±0.2 °C up to 450 or down to −80 °C.


Thermometers have been built which utilize a range of physical effects to measure temperature. Temperature sensors are used in a wide variety of scientific and engineering applications, especially measurement systems. Temperature systems are primarily either electrical or mechanical, occasionally inseparable from the system which they control (as in the case of a mercury-in-glass thermometer). Thermometers are used within roadways in cold weather climates to help determine if icing conditions exist. Indoors, thermistors are used in climate control systems such as air conditioners, freezers, heater
A heater is an object that emits heat or causes another body to achieve a higher temperature. In a household or domestic setting, heaters are usually appliances whose purpose is to generate heating...

s, refrigerator
A refrigerator is a common household appliance that consists of a thermally insulated compartment and a heat pump that transfers heat from the inside of the fridge to its external environment so that the inside of the fridge is cooled to a temperature below the ambient temperature of the room...

s, and water heater
Water heating
Water heating is a thermodynamic process using an energy source to heat water above its initial temperature. Typical domestic uses of hot water are for cooking, cleaning, bathing, and space heating...

s. Galileo thermometers are used to measure indoor air temperature, due to their limited measurement range.

Alcohol thermometer
Alcohol thermometer
The Alcohol thermometer or spirit thermometer is an alternative to the mercury-in-glass thermometer, and functions in a similar way. But unlike mercury-in-glass thermometer, the contents of an alcohol thermometer are less toxic and will evaporate away fairly quickly...

s, infrared thermometer
Infrared thermometer
Infrared thermometers infer temperature using a portion of the thermal radiation sometimes called blackbody radiation emitted by the object of measurement. They are sometimes called laser thermometers if a laser is used to help aim the thermometer, or non-contact thermometers to describe the...

s, mercury-in-glass thermometers, recording thermometer
Recording thermometer
A recording thermometer is a type of thermometer that records temperature changes over a period of time.The recording thermometer uses a strip that coils and uncoils as the temperature changes. One end of the strip is attached to a long, light metal lever that holds a special pen. Tiny movement of...

s, thermistor
A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor...

s, and Six's thermometers are used outside in areas which are well-exposed to the elements at various levels of the Earth's atmosphere and within the Earth's oceans is necessary within the fields of meteorology
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...

 and climatology
Climatology is the study of climate, scientifically defined as weather conditions averaged over a period of time, and is a branch of the atmospheric sciences...

. Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

 use thermometers and hygrometer
A hygrometer is an instrument used for measuring the moisture content in the environmental air, or humidity. Most measurement devices usually rely on measurements of some other quantity such as temperature, pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed...

s to determine if atmospheric icing
Atmospheric icing
Atmospheric icing occurs when water droplets in the atmosphere freeze on objects they contact. This can be extremely dangerous to aircraft, as the built-up ice changes the aerodynamics of the flight surfaces, which can increase the risk of a subsequent stalling of the airfoil...

 conditions exist along their flight path
Airway (aviation)
In aviation, an airway is a designated route in the air. Airways are laid out between navigational aids such as VORs, NDBs and Intersections ....

, and these measurements are used to initialize weather forecast models
Weather forecasting
Weather forecasting is the application of science and technology to predict the state of the atmosphere for a given location. Human beings have attempted to predict the weather informally for millennia, and formally since the nineteenth century...

. Thermometers are used within roadways in cold weather climates to help determine if icing conditions exist and indoors within climate control systems.

Bi-metallic stemmed thermometers, thermocouple
A thermocouple is a device consisting of two different conductors that produce a voltage proportional to a temperature difference between either end of the pair of conductors. Thermocouples are a widely used type of temperature sensor for measurement and control and can also be used to convert a...

s, infrared thermometers, and thermisters are handy during cooking in order to know if meat has been properly cooked
Meat thermometer
thumb|A meat thermometer with a dial. Notice the markings for each type of meatA meat thermometer is a thermometer used to measure the internal temperature of meat, especially roasts and steaks, and other cooked foods. The degree of "doneness" of meat correlates closely with the internal...

. Temperature of food is important because if it sits within environments with a temperature between 5 and 57 °C (41 and 134.6 F) for four hours or more, bacteria can multiply leading to foodborne illness
Foodborne illness
Foodborne illness is any illness resulting from the consumption of contaminated food, pathogenic bacteria, viruses, or parasites that contaminate food, as well as chemical or natural toxins such as poisonous mushrooms.-Causes:Foodborne illness usually arises from improper handling, preparation, or...

es. Thermometers are used in the production of candy
Candy thermometer
A candy thermometer, also known as a sugar thermometer, is a thermometer used to measure the temperature and therefore the stage of a cooking sugar solution. These thermometers can also be used to measure hot oil for deep frying.There are several kinds of candy thermometers available...


Medical thermometers such as mercury-in-glass thermometers, infrared thermometers, pill thermometer
Pill thermometer
A pill thermometer is an ingestible thermometer that allows a person's core temperature to be continuously monitored. It was developed by NASA in collaboration with Johns Hopkins University for use with astronauts...

s, and liquid crystal thermometer
Liquid crystal thermometer
A liquid crystal thermometer or plastic strip thermometer is a type of thermometer that contains heat-sensitive liquid crystals in a plastic strip that change color to indicate different temperatures....

s are used within health care
Health care
Health care is the diagnosis, treatment, and prevention of disease, illness, injury, and other physical and mental impairments in humans. Health care is delivered by practitioners in medicine, chiropractic, dentistry, nursing, pharmacy, allied health, and other care providers...

 to determine if individuals have a fever
Fever is a common medical sign characterized by an elevation of temperature above the normal range of due to an increase in the body temperature regulatory set-point. This increase in set-point triggers increased muscle tone and shivering.As a person's temperature increases, there is, in...

 or are hypothermic
Hypothermia is a condition in which core temperature drops below the required temperature for normal metabolism and body functions which is defined as . Body temperature is usually maintained near a constant level of through biologic homeostasis or thermoregulation...


Liquid crystal thermometers can also be used to measure the temperature of water in fish tanks.

Fiber Bragg grating
Fiber Bragg grating
A fiber Bragg grating is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. This is achieved by adding a periodic variation to the refractive index of the fiber core, which generates a...

 temperature sensors are used within nuclear power
Nuclear power
Nuclear power is the use of sustained nuclear fission to generate heat and electricity. Nuclear power plants provide about 6% of the world's energy and 13–14% of the world's electricity, with the U.S., France, and Japan together accounting for about 50% of nuclear generated electricity...

 facilities to monitor reactor core temperatures and avoid the possibility of nuclear meltdown
Nuclear meltdown
Nuclear meltdown is an informal term for a severe nuclear reactor accident that results in core damage from overheating. The term is not officially defined by the International Atomic Energy Agency or by the U.S. Nuclear Regulatory Commission...


Various types of thermometer

  • Beckmann differential thermometer
    Beckmann thermometer
    A Beckmann thermometer is a device used to measure small differences of temperature, but not absolute temperature values. It was invented by Ernst Otto Beckmann , a German chemist, for his measurements of colligative properties in 1905...

  • Bi-metal mechanical thermometer
  • Coulomb blockade thermometer
    Coulomb blockade
    In physics, a Coulomb blockade , named after Charles-Augustin de Coulomb's electrical force, is the increased resistance at small bias voltages of an electronic device comprising at least one low-capacitance tunnel junction. Because of the CB, the resistances of devices are not constant at low bias...

  • Heat meter
    Heat meter
    thumb|right|Heat meter in a [[District heating substation]] in a residential neighborhood. Right in whit-blue: the calculator; in the center in bronze: the ultrasonic flow meter...

  • Phosphor thermometry
    Phosphor thermometry
    Phosphor thermometry is an optical method for surface temperature measurement. The method exploits luminescence emitted by phosphor material. Phosphors are fine white or pastel-colored inorganic powders which may be stimulated by any of a variety of means to luminesce, i.e. emit light...

  • Pyrometer
    A pyrometer is a non-contacting device that intercepts and measures thermal radiation, a process known as pyrometry.This device can be used to determine the temperature of an object's surface....

  • Quartz thermometer
    Quartz thermometer
    The quartz thermometer is a high-precision, high accuracy temperature sensor. It measures temperature by measuring the frequency of a quartz crystal oscillator. The oscillator contains a specially cut crystal that results in a linear temperature coefficient of frequency, so the measurement of the...

  • Rectal thermometry
    Rectal thermometry
    Rectal thermometry is an umbrella term covering the practice, widely used in modern medicine and science, of taking a mammal's temperature by inserting a thermometer into the aforementioned mammal's rectum via the anus. This is generally regarded as the most accurate means of temperature-taking,...

  • Reversing thermometer
    Reversing thermometer
    Unlike most conventional mercury thermometers, a reversing thermometer is able to record a given temperature to be viewed at a later time. If the thermometer is flipped upside down, the current temperature will be shown until it is turned upright again...

  • Silicon bandgap temperature sensor
    Silicon bandgap temperature sensor
    The silicon bandgap temperature sensor is an extremely common form of temperature sensor used in electronic equipment. Its main advantage is that it can be included in a silicon integrated circuit at very low cost...

  • Temperature strip
    Temperature strip
    Temperature strips are flexible, often disposable visual indicators used to quickly and easily take and monitor surface temperature. They are used in laboratory, food and beverage, hospital and home applications, and often use a gradient color-scheme to communicate a safe or out-of-tolerance...

  • Wearable thermometer
    Wearable thermometer
    These are the thermometers that can be worn. These thermometers continuously measure the temperature as they are always on body. Some of these are bluetooth enabled...

Further reading

  • Middleton, W. E. K. (1966). A history of the thermometer and its use in meteorology. Baltimore: Johns Hopkins Press. Reprinted ed. 2002, ISBN 0801871530.
  • History of the Thermometer

External links

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