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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 youtube.com/crashcourse.
And as always, thanks for watching this episode
of SciShow.
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