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. The key thing to remember about the Seebeck effect is that the size of the voltage or current created depends only on the type of metal (or metals) involved and the temperature difference. You don’t need a junction between different metals to produce a Seebeck effect: only a temperature difference. In practice, however, thermocouples do use metal junctions.

Measuring temperatures with a thermocouple

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.

What are thermocouples like in practice?

All we really care about is one of the two junctions—the one measuring the unknown temperature. So when you see a photograph or an illustration of a thermocouple, that’s usually all that’s shown: A typical thermocouple, showing the junction of metals. NASA photo. What you see here is the part of a thermocouple that measures the unknown temperature. In practice, it’s wired into a larger circuit, something like the one below, with the two outputs feeding into an electronic voltage amplifier. This increases the very small voltage difference between the two parts of the circuit (the upper and lower paths in this diagram) so it can be measured more accurately. To be absolutely clear, the instrument you see in the photo up above is just the extreme left part of the artwork below:

How a typical thermocouple is wired into a circuit with a voltage amplifier.

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).

Line artwork showing the main components of a typical two-wire thermocouple, including the insulation.

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. Read More