(classical music)
- Hey there, I'm Josh Clark, and this is Brain Stuff.
And this is the particular brain stuff
where I explain to you what a Crookes radiometer is,
and I even explain how it works.
So if you'd been alive in 1870,
you would know that the Crookes radiometer
was the must-have toy of the year.
It was marketed as something called the Light Mill,
which will make a lot of sense in a second.
So, for those of you who've never seen a Crookes radiometer,
basically, it looks like a paper-thin,
see-through glass light bulb.
It's sealed with a partial vacuum inside.
The shaft coming up, and arrayed around that shaft,
four little vanes on a frictionless rotor.
The vanes have a dark side and a silvery, reflective side.
Okay, got it?
That's the Crookes radiometer.
That's what it looks like.
And, since it's sealed off in a glass bulb,
it shouldn't be subject to air currents.
In fact, its inventor, Sir William Crooke,
came up with this whole thing for that very reason.
See, he was looking to measure thallium,
which is very, very lightweight,
without air currents messing with his measurements.
He figured out that when you put a Crookes radiometer
in the presence of sunlight,
the weather vane starts to inexplicably move around.
Now, Crookes could have very easily just shouted,
"Witchcraft, witchcraft!" and hid under his bed,
but he didn't because he was a scientist,
so he sought to explain it,
and he came up with a proposal called the pressure of light.
See, what Crookes thought was going on,
is the sunlight hitting the weather vane
was pushing it around.
Basically that photons, the tiniest packets of light,
were hitting the reflective surfaces
and causing the vane to spin, get it?
But it was wrong, and here's why.
If Crookes had been correct,
the reflective side of each weather vane
should have been pushed along,
and the vane would have spun in the direction
of the silver side.
Well, anybody who's ever messed with a Crookes radiometer
knows that, when you put it in sunlight,
it spins the opposite direction,
with the darker side of each vane leading the way.
So, what's going on here?
What's the deal?
Is it witchcraft after all?
Maybe.
See, in 1879, a guy named Osborne Reynolds,
which is a pretty awesome name,
proposed what is considered
the blue-ribbon hypothesis explanation
of what happened with the Crookes radiometer.
And what he proposed was thermal creep,
or thermal transpiration.
Now, remember, Crookes radiometer is a glass bulb
that's sealed off, and it's sealed
because it's containing a partial vacuum inside.
Now, because it's a partial vacuum, thermal creep
is allowed to occur.
It also means that the gasses inside,
any air that's still left, is evenly distributed.
That's how nature loves everything, even Steven.
But when you place the Crookes radiometer in sunlight,
you're introducing heat in the form of light,
and when you do that, remember that each of the four vanes
has a silvery, reflective side, and a darker black side,
and that darker black sides tend to absorb light,
enhance heat, which means that you have one side of a vane
that's hotter than the other side.
And when that happens, nature likes to even things out,
remember, by sending colder air on the silvery side
around to the darker side to cool it off, cool things down.
When it does that, the balance of gasses changes.
It builds up on the darker side,
increasing the air pressure there,
and decreasing the air pressure on the silvery side, got it?
That's part one of thermal creep.
The second part is that, as the cold air moves,
air particles move around to the warmer side,
and sometimes they displace some of the warmer molecules,
which go to the other side.
Now, by definition, warmer air molecules have more energy.
They're more excited, and so they
strike the vane with more force.
They're striking the black side of the vane,
which, on the other side, meets very little resistance
because there's lower air pressure,
and the vane spins around, with the dark side leading.
Hence, the Crookes radiometer is explained
once and for all by me.
And, if this didn't make your head explode,
and made your thirst for Crookes radiometers huge,
you can get them on the internet.
We want to hear what you think about Crookes radiometers.
You should leave us a comment in the comment section below.
Say, "Hey."
Say, "Crookes radiometers are awesome."
Or, "Crookes explanations are full of it."
Or, "It is witchcraft after all."
Who knows, but we want to hear from you either way.
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