Thermometer

A thermometer is a device that measures temperature (the hotness or coldness of an object) or temperature gradient (the rates of change of temperature in space). It has two important parts: a temperature sensor, like the bulb in a mercury-in-glass thermometer or the sensor in an infrared thermometer, and a way to show the temperature as a number, such as the markings on a glass thermometer or the digital display on an infrared model.

Mercury thermometer (mercury-in-glass thermometer) for measurement of room temperature.

Thermometers are used in many places. They help watch over processes in technology and industry, are important in meteorology to understand weather, are used in medicine with a medical thermometer, and are valuable tools in scientific research. This makes them useful in everyday life and many important fields.

A standard scale

Main articles: Temperature and Temperature measurement

History

It is hard to say exactly when the thermometer was first made because records from long ago are incomplete and unclear. The thermometer did not appear suddenly; instead, it developed slowly over time with many small improvements.

Ancient developments

Early ideas about air and temperature helped inspire the creation of thermometers much later.

Philo of Byzantium

Fludd's figure of Philo's experiment

Around the year 300 BC, Philo of Byzantium described an experiment. He used a tube placed in a container of liquid. The other end of the tube was connected to a hollow sphere sealed with air. When the air in the sphere was warmed, such as by a candle or sunlight, the air expanded and pushed out, creating bubbles in the liquid. When the air cooled, it sucked liquid up into the tube. The position of the liquid showed whether the air was getting warmer or cooler.

Translations of Philo's work were later used by Robert Fludd around 1617 for his air thermometer.

Hero of Alexandria

In his book Pneumatics, Hero of Alexandria (10–70 AD) described a device that used the sun's heat to move water, building on ideas similar to Philo's. His work was later read by many scientists, including Galileo Galilei, in the late 1500s.

First temperature scale with a fixed point

The Roman Greek physician Galen introduced important ideas for measuring temperature. He suggested that heat or cold could be measured in “degrees” and proposed a fixed reference temperature using a mix of ice and boiling water. In the 1500s, physician Johann Hasler used Galen's ideas to create scales for mixing medicines.

Hasler's temperature scale showing degrees of body temperature based on an individual's latitude.

Late Renaissance developments

Thermoscope

Main article: Thermoscope

In the late 1500s and early 1600s, scientists like Galileo Galilei and Santorio Santorio created devices with a glass bulb filled with air, connected to a tube with some water. When the air in the bulb warmed or cooled, the water level in the tube rose or fell, showing changes in heat. These early devices, called thermoscopes, could show changes in heat but did not have numbered scales.

Air thermometer

The main difference between a thermoscope and a thermometer is that a thermometer has a scale. Some possible inventors of the thermometer include Galileo, Santorio, Cornelis Drebbel, or Robert Fludd. Although Galileo is often credited, no documents show he actually made one.

The first diagram showing a scale on such a device was made by Santorio Santorio in 1625. This device, an air thermometer, used a tube with a bulb at the top and water at the bottom. The water level changed with the expansion and contraction of air.

Fifty-degree thermometers from the mid-17th century on exhibit at the Museo Galileo with black dots representing single degrees and white represented 10-degree increments; used to measure atmospheric temperatures

Coining of thermometer

The word “thermometer” first appeared in 1624 in a book by Jean Leurechon. It comes from Greek words meaning “warmth” and “measure.”

Sealed liquid-in-glass thermometer

Precision thermometry

A medical mercury-in-glass maximum thermometer.

In 1714, Daniel Gabriel Fahrenheit invented a reliable thermometer using mercury instead of alcohol or water mixtures. In 1724, he created a temperature scale that now carries his name. In 1742, Anders Celsius proposed a scale with zero at the boiling point and 100 degrees at the freezing point of water, though later it was reversed. René Antoine Ferchault de Réaumur invented an alcohol thermometer in 1730, but it was less reliable than Fahrenheit’s.

The first doctor to use thermometers in medical care was Herman Boerhaave. In 1866, Thomas Clifford Allbutt invented a fast-acting medical thermometer that gave results in five minutes instead of twenty.

Registering

Older thermometers could not remember the temperature after being moved. For example, if you took a thermometer out of hot water to read it, the reading would quickly change to match the cooler air around you. Modern thermometers can "register" or remember the temperature, allowing you to read it later or in a more comfortable place.

Some thermometers keep track of the highest or lowest temperature reached until someone resets them. Others can record the temperature at a specific time. Many thermometers today use electronic parts to show the temperature on a digital screen or send the information to a computer.

Physical principles of thermometry

Thermometers can be two types: absolute and empirical. Absolute thermometers use a special temperature scale, while empirical thermometers compare temperatures but might not match the absolute scale exactly. However, all thermometers agree on which object is hotter or colder.

Various thermometers from the 19th century.

One important idea in thermometry is the "zeroth law of thermodynamics," which helps us understand how temperatures relate to each other. It tells us that if two objects have the same temperature, a third object in contact with both will have the same temperature change, no matter which of the first two it touches first.

There are different ways to build thermometers, often using special materials that change in predictable ways with temperature. For example, mercury expands when heated, which we can see in a glass thermometer. Some thermometers use the way materials radiate energy, which also changes with temperature, making them useful for measuring very high temperatures.

Primary and secondary thermometers

A thermometer can be called primary or secondary based on how it measures temperature. Primary thermometers measure a property of matter so well that the temperature can be calculated directly. Examples include thermometers that use the behavior of gases, the speed of sound, or electrical signals.

Secondary thermometers are more commonly used because they are easier to use and often more sensitive. However, they need to be checked against a primary thermometer at certain temperatures to make sure they are accurate. These check points, like specific temperature changes, happen at the same temperature every time.

Calibration

Thermometers can be checked to make sure they are correct by comparing them to other thermometers that are already known to be right, or by using special points on the temperature scale. The most well-known points are when pure water melts and when it boils. (Remember, the boiling point of water changes depending on the pressure, so this needs to be controlled.)

The old way to mark a scale on a thermometer had three steps:

Mercury-in-glass thermometer

Put the sensing part in a mix of pure ice and water at normal air pressure and mark where it shows when it settles.
Put the sensing part in steam at standard atmospheric pressure and mark where it shows again.
Split the space between these two marks into equal parts based on the temperature scale being used.

Other points used in the past included body temperature and a very cold mix of salt and ice. But these were replaced by the International Temperature Scale of 1990. Today, makers often use a special bath or block where the temperature stays the same and check other thermometers against it. For many modern devices, calibration just means setting a value to help change an electronic signal into a temperature reading.

Precision, accuracy, and reproducibility

The "Boyce MotoMeter" radiator cap on a 1913 Car-Nation automobile, used to measure temperature of vapor in 1910s and 1920s cars.

The precision of a thermometer is how small the temperature changes it can show. Some thermometers can only show changes of about 10 °C, while others, like clinical or electronic ones, can show changes as small as 0.1 °C. Very special tools can even show changes to one thousandth of a degree. But remember, showing small changes doesn’t mean the temperature is exactly right.

A thermometer that is checked against a known temperature point will give the right reading at that point. To find temperatures between these known points, we usually guess in a straight line. Different thermometers might guess a little differently, especially far from the known points. For example, a mercury thermometer and a platinum resistance thermometer might not agree perfectly around 50 °C.

For many uses, it’s important that the same thermometer (or similar ones) gives the same reading each time for the same temperature. This makes experiments and processes consistent. If a thermometer is checked and used the same way as others of its type, its readings can be trusted, even if it’s not perfectly exact.

Separated columns are often a problem in both alcohol and mercury thermometers, and they can make a temperature reading inaccurate.

For example, a platinum resistance thermometer checked against national standards at five points (−18, 0, 40, 70, 100 °C) can be trusted to be within ±0.2 °C of the true temperature.

According to British Standards, liquid-in-glass thermometers that are properly checked, used, and kept in good condition can be very accurate, with an uncertainty of ±0.01 °C between 0 and 100 °C. Outside this range, the uncertainty grows: ±0.05 °C up to 200 °C or down to −40 °C, and ±0.2 °C up to 450 °C or down to −80 °C.

Main articles: scales of temperature, interpolation, platinum, capillary tube, temperature measurement, resistance thermometer

Indirect methods of temperature measurement

Main article: Temperature measurement § Technologies

There are many ways to measure temperature without using a standard thermometer. One way is by using how much materials grow when they get hotter. For example, two metals that bend in different ways when heated can show temperature on a dial. Liquids like alcohol or mercury also grow when heated and are used in common thermometers.

An infrared thermometer is a kind of pyrometer (bolometer).

Another way is by looking at how objects give off light or heat from a distance. Very hot objects glow, and their color can tell us how hot they are. Some special tools can even measure temperature by watching how light changes, which is useful for things that are too far away to touch.

Electrical changes in materials can also show temperature. Some metals change their resistance when heated, and this can be measured to find the temperature. Other tools use tiny changes in electrical patterns to sense heat, especially in very cold or very hot places.