Thermometer

A thermometer is a device that measures temperature (how hot or cold something is). It has two main parts: a 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.

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

See also: Scale of temperature

When we measure temperature with a thermometer, we need a way to compare the readings. Today, there is a way to measure temperature called the thermodynamic temperature. Countries use agreed-upon temperature scales that follow this method. The latest official scale is the International Temperature Scale of 1990. This scale works from very cold to quite hot temperatures.

History

See also: Timeline of temperature and pressure measurement technology

It is hard to say when the thermometer was first made because old records are incomplete. The thermometer developed slowly with many small improvements.

Ancient developments

Early ideas about air and temperature helped inspire thermometers later.

Philo of Byzantium

Around the year 300 BC, Philo of Byzantium described an experiment. He used a tube in a container of liquid. The other end of the tube was connected to a hollow sphere with air inside. When the air in the sphere warmed up, it expanded and pushed liquid out. When the air cooled, it pulled liquid back in. The liquid level showed if the air was warming or cooling.

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. This built 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 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.

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.

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

See also: Alcohol thermometer

Early devices could also measure air pressure because they used air. In 1629, Joseph Solomon Delmedigo described a sealed tube with liquid (brandy) inside, which was an early version of today’s thermometer. Later, Ferdinando II de' Medici made the first modern thermometer using liquid that worked independently of air pressure.

Precision thermometry

See also: Precision thermometry, Fahrenheit scale, Celsius scale, Mercury-in-glass thermometer (mercury thermometer), Medical thermometer, Pyrometer, and Infrared 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 come in two main types: absolute and empirical. Absolute thermometers use a special temperature scale. Empirical thermometers compare temperatures but might not match the absolute scale exactly. But all thermometers can tell us which object is hotter or colder.

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

There are many ways to make thermometers, often using special materials that change in known ways with temperature. For example, mercury gets bigger when heated, which we can see in a glass thermometer. Some thermometers use how materials give off energy, which also changes with temperature. These are useful for measuring very high temperatures.

Primary and secondary thermometers

A thermometer can be 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. We can use special points on the temperature scale to do this. The most well-known points are when pure water melts and when it boils.

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

1. Put the sensing part in a mix of pure ice and water and mark where it shows.
2. Put the sensing part in steam at standard atmospheric pressure and mark where it shows.
3. Split the space between these two marks into equal parts.

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 means setting a value to help change an electronic signal into a temperature reading.

Precision, accuracy, and reproducibility

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 can show changes as small as 0.1 °C. Very special tools can show even smaller changes.

A thermometer that is checked against a known temperature 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.

For many uses, it’s important that the same thermometer (or similar ones) gives the same reading each time for the same temperature. This helps make experiments and processes consistent. If a thermometer is checked and used the same way as others of its type, its readings can be trusted.

For example, a platinum resistance thermometer checked against national standards at five points can be trusted to be very close to 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.

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 watching how materials change 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.

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

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

Applications

See also: List of temperature sensors

Thermometers are tools that help us measure how hot or cold something is. We use them in many places, like science labs, factories, and our homes. For example, they help keep our food safe by making sure it’s not too warm or too cold. Thermometers are also used inside buildings to control the temperature, like in air conditioners, freezers, and heaters.

Some thermometers are used to check our body temperature when we are sick. Others are used in factories and power plants to help everything run safely. There are also very small thermometers that scientists use to study tiny things.