Vocab Lesson: Thermocouples

Vocab Lesson: Thermocouples

Welcome to Vocab Lesson, Gizmodo’s new weekly column on words – the ones you’ve heard, but can’t quite define, or the ones you haven’t, but might like to hear about. This week’s lesson: thermocouples! (Huh?)

You almost certainly own a thermocouple. You probably have at least a few and may well command a small army of them every day – in your oven, your fridge, your boiler, your heater and your microwave, to name a few. Thermocouples are thermometers, yes. But that doesn’t do them justice: They’re the thermometers that go where other thermometers can’t. They’re as happy in a scorching steel mill as they are in the base of your electric kettle. They’re as ready to probe your rib roast as they are to orbit Saturn. They’re thermocouples, and they make that wimpy tube of mercury in your medicine cabinet look like a joke.

What the Word Means: From WordNet: (n) thermocouple, thermocouple junction, a kind of thermometer consisting of two wires of different metals that are joined at both ends; one junction is at the temperature to be measured and the other is held at a fixed lower temperature; the current generated in the circuit is proportional to the temperature difference

What THAT Means: So again: A thermocouple is a thermometer, but for lack of a better word, it’s cooler. (Get it? Eh?) Instead of delicate glass tubes and chambers of mercury or alcohol, they’re built from metal, which means they can survive where a dainty borosilicate tube can’t. In fact, they can basically survive – and keep giving temperature readouts – until they melt.

Physically, a thermocuple is just two wires made out of different metals, joined at the ends. At one of the junctions, there’s a current sensor. So how the hell is THAT a thermometer? Thermoelectrics, yo!

About 200 years ago, an Estonian dude named Thomas Seebeck announced that he had discovered… something. He observed that if he heated up one side of a ring made from two pieces of metal – in this case, half bismuth, half copper – it made his compass go nuts. Reasonably, he thought the heat magnetised the ring, and he was sort of right: It wasn’t that the difference in temperatures was causing magnetism, strictly speaking. It was producing an electrical current. Our mistaken Estonian hero’s discovery of what he called “thermo-magnetism” actually turned out to be the basis of pretty much all of thermoelectrics, which is a pretty awesome consolation prize, legacy-wise.

It didn’t take too long for scientists to start to notice that with any given thermocouple, certain temperatures correlated to certain amounts of electricity. They figured out how to measure this. They scribbled out some equations and maybe some charts: This much heat gives us that much current. Voila. A new thermometer.

All this temperature-reading stuff is great, but think about what Seebeck actually discovered: In this pair of wires, heat was creating electricity. Does that mean that every thermocouple is also a teensy little thermoelectric generator?

Yes. In fact, if you join a whole bunch of them together into a chain, you get something called a thermopile.

Stick said thermopile next to a steady source of heat energy – say, decaying radioactive material – and you end up with something like this; a radioisotope thermoelectric generator sent along with the Cassini spacecraft in lieu of solar panels, which wouldn’t have been much use all the way out there in dark-as-hell, bumfuck Saturntown.

And so there you have it! Unsung, heatsoaked heroes of land and space, thermocouples are everywhere. Now you know.

If you’d like to hear more about a strange tech word or phrase, send a request along. We’ll look it up for you! Probably. Cassini thermopile image courtesy of NASA