BrainStuff Classics: How Does Crooke's Radiometer Work?

Crooke's radiometer is a device containing vanes that appear to spin because they're pushed by light itself, but that's not quite right. Learn what's really happening in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:01):
Welcome to brain Stuff production of iHeart Radio. Hey brain Stuff,
Lauren bog Obam here. Today's episode is a classic from
our erstwhile host, Christian Sager. In this one, we break
down the surprising inner workings of an old conversation piece,
Crooks Radiometer, which is one of those glass thingies with
the little black and white sided spinny things inside. I'll

(00:25):
let Christian explain, Hey brain Stuff, it's Christian Sager. Crooks
Radiometer was one of the most sensational toys of the
eighteen seventies, but no one had any idea how it worked,
which caused much scientific debate. In a two thousand four
article in New Scientists, Andre Larazza of the Naval post

(00:48):
Graduate School in Monterey, California, went so far to say this,
even today, most physicists think they know how it works,
while few actually do. Well. Today, we're not only going
to explain how Crooks Radiometer works, but we're actually going
to explain what it is. Okay, you're ready, this is

(01:09):
what it is. It's a glass bulb with four veins
suspended inside with a good but not perfect vacuum. Each
vein is blackened on one side and silvered on the
other They are all attached to a rotor which is
balanced on a vertical support that turns with very little friction.

(01:29):
When you shine light on the veins, they spin. And
it started out as a toy, but it's now marketed
as like a conversation piece, and it's often called a
light mill. Maybe you've seen one. So how is this
thing invented? Well? Even Crooks, the guy who invented it,
he didn't understand how it worked when he built the

(01:49):
first radiometer in eighteen seventy three, and it was a
byproduct of his chemical research. Came about while he was
weighing samples of thallium in a vacuum chamber to deduce
the effect of air currents. Krook noticed when sunlight shone
on the balance, his measurements were disturbed. Warm samples appeared
to weigh less than cold ones. Investigating further, he discovered

(02:13):
a black surface was repelled more strongly than a silver one.
Based on these findings, he built the first radiometer, which
became a popular toy and novelty item that same year.
Krooks suggested that the mill spun because of quote the
pressure of light, and it was pushing it like a
water mill. But the light falling on the black side

(02:34):
should have been absorbed, while the light falling on the
silver side would be reflected, causing the radiation pressure to
push on the silver side. But Krook's radiometer was pushing
on the black side, meaning it was turning the wrong
way from his explanation, So how does this thing actually work? All? Right,
here we go. The vacuum in the radiometer is important.

(02:57):
It has to be just right for the spin to work.
If there's no vacuum, there's too much drag for the
veins to move. If there's a near perfect vacuum, the
veins won't spin unless they're held in place with the
impediment of friction. But if the veins have frictionless support
from the rotor and the vacuum is good but incomplete,

(03:20):
then thermal transpiration takes place and it looks like the
light is pushing against the black sides, but in fact
the black side is moving away from the light. Osborne
Reynolds provided the correct solution in eighteen seventy nine. He
explained that thermal transpiration, or I like to refer to

(03:41):
it as thermal creep which some other people use was
the flow of gas caused by a temperature difference on
either side of the veins. If gas is originally at
the same pressure on each side, it flows from the
colder to the hotter side, resulting in higher pressure on
the hotter side. In the case of the veins, the

(04:03):
faster molecules from the warm side strike their edge, imparting
more force than the cold molecules and moving the vein
away from the warmer guests. Today's episode was written by
Christian and produced by Tyler Clain. Brain Stuff is production
of I Heart Radio's How Stuff Works. For more on

(04:24):
this and lots of other topics, visit our home planet
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