by Chris Woodford. Last updated: December 12, 2013.
Now that's what I call hot! But how hot is it, exactly? If you want to measure the temperature of something as hot as a volcano, an ordinary thermometer isn't any use. Stick the bulb of a mercury thermometer into volcanic lava (which can be well over 1000°C or 1800°F) and you'll get a surprise: the mercury inside will instantly boil (it turns from liquid to gas at a mere 356°C or 674°F) and the glass itself might even melt (if the lava is really hot)! Try measuring something super-cold (like liquid nitrogen) with a mercury thermometer and you'll have the opposite problem: at temperatures below −38°C/38°F, mercury is a solid lump of metal! So how do you measure really hot or cold things? With a cunning pair of electric cables called a thermocouple. Let's take a closer look at how it works!
Photo: Measuring the temperature of volcanic lava with a thermocouple. This photo was taken at Hawaii Volcanoes National Park following the eruption of Kilauea Volcano in 1983. Photo by J.D. Griggs courtesy of US Geological Survey (USGS).
What's the connection between electricity and heat?
Have you noticed that when we talk about conduction in science we can be referring to two things? Sometimes we mean heat and sometimes we mean electricity. A metal like iron or gold conducts both heat and electricity really well; a material like a plastic doesn't conduct either of them very well at all. There is a connection between the way a metal conducts heat and the way it conducts electricity. If you've read our main article on electricity, you'll know electric current is carried through metals by tiny charged particles inside atoms called electrons. When electrons "march" through a material, they haul electricity with them a bit like ants carrying leaves. If electrons are free to carry electrical energy through a metal, they're also free to carry heat energy—and that's why metals that conduct electricity well are also good conductors of heat. (Things aren't quite so simple for nonmetals, however, because heat travels through them in other, more complex ways. But for the purposes of understanding thermocouples, metals are all we need to consider.)
Thomas Seebeck and the thermoelectric effect
Suppose you stick an iron bar in a fire. You'll know you have let go of it quite quickly because heat will be traveling up the metal from the fire to your fingers. But did you realize that electricity is traveling up the bar as well? The first person to properly cotton on to this idea was German physicist Thomas Seebeck (1770–1831), who found that if two ends of a metal were at different temperatures, an electric current would flow through it. That's one way of stating what's now known as the Seebeck effect or thermoelectric effect. Seebeck found things got more interesting as he explored further. If he connected the two ends of the metal together, no current flowed; similarly, no current flowed if the two ends of the metal were at the same temperature.
Seebeck repeated the experiment with other metals and then tried using two different metals together. Now if the way electricity or heat flows through a metal depends on the material's inner structure, you can probably see that two different metals will produce different amounts of electricity when they're heated to the same temperature. So what if you take an equal-length strip of two different metals and join them together at their two ends to make a loop. Next, dip one end (one of the two junctions) in something hot (like a beaker of boiling water) and the other end (the other junction) in something cold. What you find then is that an electric current flows through the loop (which is effectively an electric circuit) and the size of that current is directly related to the difference in temperature between the two junctions.
Photo (right): A typical thermocouple. You can see clearly here how two different metals have been joined together. Photo by courtesy of NASA Glenn Research Center (NASA-GRC).
How a thermocouple works
See where we're going with this? If you measure a few known temperatures with this metal-junction device, you can figure out the formula—the mathematical relationship—that links the current and the temperature. That's called calibration: it's like marking the scale on a thermometer. Once you've calibrated, you have an instrument you can use to measure the temperature of anything you like. Simply place one of the metal junctions in a bath of ice (or something else of a precisely known temperature). Place the other metal junction on the object whose temperature you want to find out. Now measure the voltage change that occurs and, using the formula you figured out before, you can precisely calculate the temperature of your object. Brilliant! What we have here is a pair (couple) of metals that are joined together (coupled) for measuring heat (which, in Greek, was called "thermos"). So that's why it's called a thermocouple.
Artwork: The basic idea of a thermocouple: two dissimilar metals (gray curves) are joined together at their two ends. If one end of the thermocouple is placed on something hot (the hot junction) and the other end on something cold (the cold junction), a voltage (potential difference) develops. You can measure it by placing a voltmeter (V) across the two junctions.
What are thermocouples used for?
Thermocouples are widely used in science and industry because they're generally very accurate and can operate over a huge range of really hot and cold temperatures. Since they generate electric currents, they're also useful for making automated measurements: it's much easier to get an electronic circuit or a computer to measure a thermocouple's temperature at regular intervals than to do it yourself with a thermometer. Because there's not much to them apart from a pair of metal strips, thermocouples are also relatively inexpensive and (provided the metals involved have a high enough melting point) durable enough to survive in pretty harsh environments.
Photos: Left: A selection of thermocouples. Right: The kind of high-temperature laboratory test for which thermocouples are invaluable. Both photos by courtesy of NASA Glenn Research Center (NASA-GRC).
A wide range of different thermocouples are available for different applications based on metals with high conductivity, such as iron, nickel, copper, chromium, aluminum, platinum, rhodium and their alloys. Sometimes a particular thermocouple is chosen purely because it works accurately for a particular temperature range, but the conditions under which it operates may also influence the choice (for example, the materials in the thermocouple might need to be nonmagnetic, noncorrosive, or resistant to attack by particular chemicals).
Find out more
On this website
- The theory and properties of thermocouple elements by Daniel D. Pollock, ASTM International, 1971. A dated but still very relevant book about the theory of how thermocouples work. Quite a bit of it is viewable on the Google Books preview.
- Practical thermocouple thermometry by Thomas W. Kerlin, Instrument Society of America, 1999.