How Thermometers Work | Thermometer Types Comparison (2023)

How Thermometers Work | Thermometer Types Comparison (1)

go throughChris WoodfordLast updated: June 13, 2022.

DDo you feel hot today or is it just me? How do we know? How do we resolve an argument if I say today is hotter than yesterday and you disagree? A simple way is to usethermometerA thermometer is a simple scientific instrument whose principle isMetalChange their behavior in very precise ways as they get hotter (get morehotvitality). Let's take a closer look at how these handy gadgets work.

Photo: This is what I call cold! This dial (pointer) thermometer shows the temperature inside my food freezer: about -30°C (inner scale) or -25°F (outer scale). This is the exact same temperature, but measured slightly differently.

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  1. liquid thermometer
  2. dial thermometer
  3. Digital Thermometer
  4. Measure extreme temperatures
  5. What is a temperature scale?
  6. learn more

liquid thermometer

The simplest thermometer is really simple! they're just skinnyGlasstubes containing a small amount of silvery liquid (usually mercury - a rather peculiar metal,liquidat normal day-to-day temperatures). As mercury heats up, it expands (increases in size), and the amount of expansion is directly related to temperature. So, if the temperature increases by 20 degrees, the mercury expands and moves on the scale twice as much as it would if the temperature was only increased by 10 degrees. All we have to do is mark the scale on the glass and we can easily calculate the temperature.

Photo: A typical thermometer consists of a liquid in a tube that rises and falls on a linear scale (an equidistant scale) marked with temperature.

How Thermometers Work | Thermometer Types Comparison (2)

How do we determine scale? It is easy to make a Celsius thermometer as it is based on ice and boiling temperaturewater. These two are calledfixed point. We know that the temperature of ice is close to 0°C, while the boiling point of water is 100°C. If we dip the thermometer into some ice, we can watch where the mercury level is and mark the lowest point on the scale, which is around 0°C. Likewise, if we dip the thermometer into boiling water, wait for the mercury to rise, and the mark will correspond to 100°C. Then all we have to do is divide the scale between these two fixed points into 100 equal steps ("Celsius" means 100 ticks) and hey, in no time, we have a working thermometer!

How Thermometers Work | Thermometer Types Comparison (3)

Photo: Alcohol Thermometer. As you can see from the red line next to the scale, these historic Dr Pepper thermometers housed in the Dublin bottling plant and the W.P. Kloster Museum in Dublin, Texas also house alcohol. Photography: Carol M. Highsmith. Photo credit: Photo collection of Lyda Hill, Texas from the Carol M. Highsmith USA Projectlibrary of congress, Print and Photo Department.

Mercury or Alcohol?

Not all liquid thermometers use mercury. If the lines you see in your thermometer are red instead of silver (as shown in the picture), your thermometer is filled with an alcoholic liquid (such as ethanol). what is the difference? Mercury is poisonous, but very safe if it is sealed inside the thermometer. However, if the glass tube of a mercury thermometer happens to break, you could be exposed to the toxic liquid inside. Therefore, alcohol thermometers are generally safer, and they can also be used to measure lower temperatures (since alcohol has a lower freezing point than mercury; pure ethanol has a freezing point of about -114°C or -170°F, while pure ethanol has a freezing point of about -40°C or -40°F for mercury).

How Thermometers Work | Thermometer Types Comparison (4)

Photo: This thermometer contains red alcohol liquid and is marked in degrees Celsius (left, bottom of image) and Fahrenheit (right, top). The current temperature is about 21°C or about 70°F. The Fahrenheit scale is named after the German physicist Daniel Fahrenheit (1686-1736), who made the first mercury thermometers in the early 18th century. The Celsius temperature scale is named after the Swedish scientist whose inventor was Anders Celeste (1701-1744).

dial thermometer

However, not all thermometers work this way. The pointer shown in our photo above has a metal pointer that moves up and down on a circular scale. Open up one of these thermometers and you'll see that the pointer is attached to a metal coil called a "coil."bimetalIt is designed to expand and bend as the temperature increases (see our articleThermostatto learn how it works). The hotter the temperature, the more the bimetal expands, pushing the needle up the farther.

How Thermometers Work | Thermometer Types Comparison (5)

Artwork: How a Dial Thermometer Works: This is the mechanism that powers a typical dial thermometer, described in a 1905 patent by Charles W. Putnam. At the top, we have the usual hands and dial arrangement. The artwork at the bottom shows what's happening on the back. A bimetallic strip (yellow) is tightly coiled and secured to the thermometer's frame and pointer. It consists of two different metals bonded together that expand by different amounts when heated. As the temperature changes, the bimetal bends more or less (shrinks or expands), and the pointer attached to it moves up or down on the scale. works fromUS Patent 798,211: ThermometerCourtesy of the US Patent and Trademark Office.

How Thermometers Work | Thermometer Types Comparison (6)

Photo: This is the coiled bimetallic strip of the actual dial thermometer (the freezer thermometer we pictured above). It's easy to see how it works: if you turn the needle by hand to a cooler temperature, the coiled ribbon will tighten; if you turn the needle to a cooler temperature by hand, the coiled ribbon will tighten; turn the pointer in the other direction and the strap will loosen.

Digital Thermometer

One problem with mercury thermometers and dial thermometers is that they take a while to react to temperature changes.ElectronicsThermometers don't have this problem: you simply touch the thermometer probe to the object you want to measure temperature, and the digital display gives you a (nearly) instant temperature reading.

How Thermometers Work | Thermometer Types Comparison (7)

Photo: Electronic medical thermometer in 2010. You place a metal probe in your mouth or elsewhere on your body, and the temperature is read from an LCD display.

Electronic thermometers work completely differently than mechanical thermometers that use mercury threads or rotating needles.resista piece of metal (easyelectricityflowing through it) changes with temperature. As the metal gets hotter,atomThe greater the internal vibration, the harder it is for current to flow and the resistance increases. Likewise, when a metal cools, the electrons move more freely and the resistance drops. (at temperatures close toabsolute zero, the theoretically possible lowest temperature is -273.15°C or -459.67°F, the resistance completely disappears, this phenomenon is calledsuperconducting.)

Electronic thermometers work by applying a voltage across a metal probe and measuring the current flowing through it. If you place the probe in boiling water, the heat of the water makes it harder for current to flow through the probe, so the resistance increases by an amount that can be accurately measured. A microchip inside the thermometer measures the resistance and converts it to a temperature measurement.

How Thermometers Work | Thermometer Types Comparison (8)

Photo: Resistance thermometer from 1912: This example of a bridge resistance thermometer was manufactured by Leeds and Northrup and was used for temperature measurement at the National Bureau of Standards (now NIST) in the early 20th century. Despite its bulky and awkward appearance, it is accurate to within 0.0001 degrees. photo byNational Institute of Standards and Technology Digital Collections, Gaithersburg, MD 20899.

The main advantage of thermometers like these is that they give instant readings on whatever scale you prefer (Celsius, Fahrenheit, or whatever). But one of their downsides is that they measure temperature moment-to-moment, so the numbers they display can fluctuate wildly, making it difficult to get an accurate reading at times.

Precision electronic thermometers (called resistance thermometers) use four resistors arranged in a diamond-shaped circuit called a Wheatstone bridge. The resistance of the fourth resistor is easy to calculate if the values ​​of three of the resistors are known. If the fourth resistor is designed in the shape of a temperature probe, such a circuit can be used as a very accurate thermometer: calculating its resistance (from its voltage and current) allows us to calculate its temperature.

Measure extreme temperatures

You need athermocouple: An ingenious device for measuring temperature by measuring electric current. If you're close enough that you can't even use a thermocouple, you could try usingpyrometer, a thermometer that calculates the temperature of an object based on its temperatureElectromagnetic radiationIt unleashes.

What is a temperature scale?

How Thermometers Work | Thermometer Types Comparison (9)

Photo: The temperature scale is linear: increasing the temperature by a certain amount will always move you the same distance on the scale. This doesn't mean that the thermometer has to be made straight like a ruler: it means that each part of the scale takes up exactly the same amount of space (or, if you prefer, the mercury, pointer, or other temperature indicator must rise or fall with temperature, moved as far as possible to indicate each new division). This dial thermometer comes fromgas boilerShows you the temperature of your central heating system in degrees Celsius using a circular (but still linear) scale.

Thermometers don't necessarily need to have graduations or numbers on them. Imagine if you were on a deserted island and you found an old thermometer in the sand with the scale and numbers worn off but otherwise working perfectly. You can still use it to know the temperature. You can use it very roughly to say things like: "The mercury levels are up about half way higher than they were yesterday, so it must be hotter today."

A better way is to put your own scale on the thermometer. First, you need to find something very cold (such as a piece of ice), put the thermometer on it, and scratch the glass to mark the mercury level. Then you can do the same with something hot (boiling water) and mark the mercury content again. We refer to these two temperature reference levels asfixed pointTo make a thermometer scale, all we have to do is divide the distance between two fixed points into many parts of equal length. That's why the Celsius thermometer gets its name: it has 100 ("minutes") sections ("grades") between the fixed points of ice and steam. What are the different temperature scales and how are they calculated?

scalefixed point


Originally 32°F (ice melting in salt) and 96°F (Daniel Fahrenheit definition).


0°C (the freezing point of water) and 100°C (the boiling point of water).


Defined by the triple point of water (solid, liquid, and vapor in equilibrium), which is 273.16 K.

ITS-90 (International Temperature Scale)

Various points are used in different parts of its range. lookITS-90more details.

How do Celsius and Fahrenheit compare?

You probably know how to convert Celsius to Fahrenheit: multiply by 9/5 (or 1.8), then add 32. To convert Fahrenheit to Celsius, do the reverse: Subtract 32 and multiply by 5/9 (or divide by 1.8, which you might think of when you hear the weather forecast give Celsius and Fahrenheit The relationship between is a bit weird and confusing because they look so different. But if you plot them on a graph (like the one below), you'll see that both scales are perfectly linear, and that temperature per For a 10°C increase, you get an 18°F increase.

How Thermometers Work | Thermometer Types Comparison (10)

Chart: The Celsius scale is shown in blue and the Fahrenheit scale is shown in red. Each point on the graph shows two equivalent measurements for a particular temperature, for example, 20°C equals 68°F. Both scales are clearly linear: a 10°C increase is equivalent to an 18°F increase.

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Books for Young Readers

  • How do we measure temperature?Author: Chris Woodford. Gareth Stevens, 2013/Blackbirch, 2005. One of my own books for young readers (ages 7-9). The focus here is on temperature as a practical form of everyday mathematics.
  • Fahrenheit, Celsius and their temperature scalesAuthor: Yoming S. Lin. PowerKIDS Press/Rosen, 2012. A historical introduction that tells the story of Daniel Fahrenheit and Anders Celeste as well as actual temperature measurement.
  • Measure it! temperatureBy Kathy Rand. Raintree, 2010. A basic introduction for children aged 7-9, including coverage of related topics such as weather and climate change.
  • Temperature: heating and coolingBy Darlene R. Steele. Picture Window Books, 2004. Another 24-page introduction for younger readers.
  • thermometerBy Adele Richardson. Capstone, 2004. A 32-page introduction with essentially the same content as this article, but for younger readers (around 6-8).

Books for Older Readers

  • Inventing Temperature: Measurements and Scientific AdvancesAuthor: Hasok Chang. Oxford University Press, 2004. The story of how people learned to measure temperature with a thermometer. It's a rather philosophical and academic book, but still very readable.
  • temperature measurementAuthor: L. Michalski. Wiley, 2001. Provides scientists and engineers with a detailed guide to accurate temperature measurement.
  • Principles and methods of temperature measurementBy Thomas Donald McGee. Wiley-IEEE, 1988. A detailed (almost 600 pages) textbook covering temperature scales and all the different types of temperature sensors, including pyrometers, thermistors, and thermocouples.


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