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Thermometers might seem like a basic instrument, but science would not be the same without them, and they helped us understand one of the most important ideas in all of science: the conservation of energy.

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[♪ INTRO].

Alright, honesty time; here comes the true-true train. You probably don’t think much about thermometers.

If you have one, it’s likely for when you’re sick, or someone in your house is sick, or maybe it’s hanging in the backyard somewhere as a decoration. But get ready to appreciate these things a lot more. Because in the 1800s, accurate thermometers helped give us one of the most important ideas in all of science: the conservation of energy.

These days, you’re told to “conserve energy” all the time, but this idea is not about turning off the lights or limiting your air conditioning. Instead, it says that energy can change forms and it is never created or destroyed, and it shapes the way we approach almost everything in physics. Lots of people, spread out over centuries, helped discover energy conservation.

But one of the most important was a physicist named James Prescott Joule. And he could not have done his groundbreaking work without a good thermometer. Essentially, Joule showed that energy can take different forms, like heat, electricity, and motion.

And while that might sound obvious today, it was a very big deal 200 years ago. In the early 1800s, physicists thought “energy” only meant “movement”, and that it was completely separate from other scientific ideas. For example, many people thought that heat was some kind of fluid.

And that made the whole concept of energy kind of useless. Since today’s scientists know that energy can take different forms, they can use it to understand all kinds of systems. Like, you wanna understand levers?

You could use complicated ideas like forces at angles making torques, or you could use the fact that balanced levers balance potential energy; energy that's stored in a system. Or, wanna know how long it’s gonna take your car to stop? If you know that brakes turn kinetic energy into thermal energy, that is, they turn movement into heat, you will work it out much faster than by calculating all of the complex molecular interactions between brakes and rotors.

To modern scientists, converting between forms of energy is a natural way of cutting past a million tiny details, and it helps them get the big picture of what's actually happening in the world. Except, it took them a while to figure this out. And that is where James Joule comes in.

He was an English physicist and experimentalist, and while he had done other research before, his adventure with energy conservation started when he found some new ways of making heat. First, he realized that you can make heat using this new-fangled “electricity” thing that everyone was talking about in the 1800s. That might be a surprise if you’re used to your laptop keeping your legs warm in the winter, but this was 1840.

Michael Faraday had invented the electric generator less than a decade earlier, when he realized that moving magnets and wires near each other forced a current through the wire. So it’s not like we knew a lot about how electricity worked at this point. In his experiment, Joule used one of those generators to prove that one part of a circuit could heat up without other parts cooling down.

If heat was a fluid like many people thought at the time, that shouldn’t have been possible. The heat should have just moved from one place to another. Then, Joule had a thought: If moving magnets and wires could turn into electricity that could turn into heat, maybe there wasn’t anything special about the magnets and the wires.

Maybe movement could be turned directly into heat, without any electricity in the middle. So he used his and other people’s experiments to calculate how much something would have to move in a tank of water to change the water’s temperature. Then, he tested his prediction by making a falling weight tug on a rope that was connected to a wheel in a tank of water.

When the weight fell, it pulled the rope, which spun the wheel. Nowadays, we’d say he converted potential energy of the weight into kinetic energy of the wheel into the thermal energy of the water. Back then, they would say that he was just wasting time.

But Joule was a rare combination of careful, clever, and persistent. He had calculated that the water’s temperature might change by half a degree Fahrenheit or less; about a quarter of a degree Celsius. And that was too tiny to reliably measure by eye with the thermometers of the day.

To get around this, Joule worked with some of the best instrument-makers in Europe to build thermometers with incredibly fine temperature differences marked on them. Then, they built a sort of traveling microscope that moved along the thermometers that let Joule quickly read between those lines. With this method, Joule claimed he could measure temperature differences as small as 1/200th of a degree Fahrenheit, which scientists then and now think was a little optimistic, but probably not by too much.

Today, it’s actually hard to confirm how good they were, because the originals were lost in a fire, and no one knows exactly how they were made. But regardless, Joule’s thermometers were certainly better than anything else around. In fact, they were so sensitive that scientists repeating Joule’s experiment with different instruments in the 1990s discovered they had to be careful that their body heat didn’t throw off measurements.

But ultimately, with his thermometers and microscope, Joule proved himself right. Turning his wheel heated up the water exactly as much as he expected, which meant that heat and motion could be converted into each other. And combined with his earlier work, it meant that heat, motion, and electricity all had something in common.

People did not accept his work right away, but Joule kept demonstrating that his results were consistent. As he worked, other scientists also started connecting his research to other discoveries, like experiments that people had been doing with steam engines, where crushing a gas heated it up. And eventually, Joule and others developed the idea of energy conservation: that heat, motion, and electricity were just some of the different forms of this thing called “energy” that could be changed between types without changing its absolute amount.

James Joule’s experiments helped found thermodynamics: the science of how energy moves around. And for his efforts, in 1882, when Joule was in his early 60’s, scientists proposed a new unit of energy: the Joule. Today, the study of physics would be, like, dramatically different without energy conservation.

This idea helps us understand thousands of systems, and it can even help us tackle the biggest challenges facing our planet, like climate change. Really, by the late 1800s, scientists were already using energy conservation to think about the Earth’s climate. They knew that some gases blocked infrared radiation, that is, light energy, better than others.

So they started wondering what would happen when humans put more of those gases into the air. Thanks to Joule and others, they didn’t have to add up the effects of each carbon dioxide and water molecule, because energy conservation let them cut to the chase:. If the atmosphere lets less energy into space, that means more must stay here on Earth, and Earth has to get warmer as a result.

So the next time you’re looking at a thermometer, take a second to appreciate that beautiful piece of engineering! It has taught us so much! It might seem like a simple instrument, but science would not be the same without it.

If you want to learn more about stories like this, you can head over to one of our sister channels: Crash Course. There, I hosted a whole series called History of Science, where we explored how we came to understand, like, basically all of the things we currently understand. You can find it by clicking the card at the end of this video, or at

And as always, thanks for watching this episode of SciShow. [♪ OUTRO].