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This toy has fascinated me since childhood.
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To me its motion is almost hypnotic.
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Here’s how it operates.
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Wet the bird’s beak thoroughly with room
temperature water — the opaque container
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makes it looked chilled, but it isn’t ... then
stand it up right . . . It’ll take a few
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seconds for it to start drinking . . . Notice
that all of the action right now takes place
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in the stem here.
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As I speed up the action you see liquid rising
and the bird rocking back and forth.
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If I return to normal speed, you can see the
bird slowly …
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very, very slowly ….
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Rock forward ...
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Until it takes a drink, which
it will do again and again.
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In this video I’ll detail the bird’s clever
engineering design, explain how it uses thermodynamics,
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and link its action to some of the greatest
and most impactful devices created by engineers.
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This toy has long history, but its current
incarnation is due to Miles V. Sullivan,
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a scientist at Bell Labs.
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He specialized in methods of manufacturing
semiconductors,
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but as a sideline invented toys.
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Its reported that this bird delighted U.S.
President Herbert Hoover, an engineer, who
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failed to figure out how it worked, and it
also defeated the great scientist Albert Einstein,
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who spent three and half months studying it.
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Its reported that he refused to take the bird
apart.
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With the benefit of hindsight, let’s start
by exploring how it works and examining the
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key engineering design aspects.
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First, let’s ask is the water ornamental
or essential?
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At first the bird acts just as if the water
were still there.
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Now let’s speed up the bird’s motion you
see at 15 minutes it is still drinking.
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At 30 still drinking.
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45 minutes still drinking.
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60 minutes still drinking.
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75 minutes still drinking.
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And five or ten minutes later, at eighty or
eight-five minutes it takes its last drink.
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The liquid still rises a bit, but it never
rises enough to make the bird tip over, which
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shows that the motion is not perpetual — as
long as there is water, the bird keeps drinking.
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Let’s look inside the bird to get an idea
of how it works.
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Underneath the bird’s hat, beak and fabric
covering lies a glass bulb, smaller than the
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bulb at the base, and also rounder.
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Now, watch as I put a few drops of isopropyl
alcohol on the top bulb to cool it.
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The liquid rapidly rises to the head, this
changes the bird’s center of gravity so
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that it will tilt forward.
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The head now fills with liquid and then …
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there ...
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… it … drinks.
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It becomes upright and the liquid drains from
the head … liquid rises again to the head and ...
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the bird drinks again.
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This cycle repeats until all of the isopropyl alcohol
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on the bird’s heat evaporates.
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Why does the liquid rise?
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The place to begin is with the bird’s manufacture.
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The bird is filled through this “tap”
— a small pipe built into the head — with
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methylene chloride dyed red, which is then
frozen, a vacuum applied to evacuate the air,
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the tap sealed (and, of course, later hidden
by the bird’s hat) . . .
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And then the methylene chloride melts:
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It turns to liquid and then
some of it evaporates (turns into vapor).
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The key to the bird’s operation is that
the vapor in the head and in the base are
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separated by the liquid in the base.
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It’s hard to see, but the tube extends into
the base, nearly reaching the bottom.
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This separates the vapor in base and the vapor
in the tube and ….
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of course, the head.
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So, at rest the pressure in these two spaces are
equal, but when the bird’s beak is wet,
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the temperature falls and, as I’ll explain
in a moment, the pressure in the head drops
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below that in the base and the liquid rises.
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Of course this liquid in the head causes the
bird to . . .
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tilt forward, to drink …
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and when it drinks, the vapor in the head and the base are connected, the pressures nearly
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equalize — a slug of vapor rises to the
top and some liquid drains from the head and
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and then the cycle repeats.
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To see the pressure equalize I’ll slow down
the bird as I tilt it forward.
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Right now the head is half full.
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When I tilt it you see a slug of vapor go
from bottom to top.
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I’ve titled it far enough forward that the
liquid in the head is below the top of the
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tube and the liquid in the base is below the
section of the tube that almost reaches the
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bottom of the bird.
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This allows the pressure to equalize, and
as the bird becomes upright the liquid returns
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to the base before the cycle starts again.
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In operation it doesn’t tilt quite this far
forward and so the pressures don’t fully equalize.
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Why, though, does the pressure in the head
drop as the temperature falls?
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You can see the answer if I shoot cool, compressed
gas across the bird’s head.
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As the cool gas strikes, you see liquid condensing
inside the head; and , as you see on the left,
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this causes the liquid in the base to rise.
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The cool gas withdraws energy as heat from
the head causing some of the methylene chloride
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vapor inside to condense - to turn into a
liquid — this decreases dramatically the
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amount of vapor in the head — liquid is
1,000 times more dense than vapor — this
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in turn lowers the pressure in the head and
causes the liquid to rise.
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I used compressed gas to cool the head because
I can control the amount of cooling; the bird,
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though, cools its head by “drinking.”
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The head is wrapped in fabric that absorbs
water.
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As I put drops on its beak you can see the
water beads up at first . . .
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then saturates the fabric and spreads rapidly across the
bird’s face.
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On the right side you can see it creeping
to back of the head.
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If I now turn the bird around … you can
see that the water has spread to the back.
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As I continue adding drops on the beak the
saturated area on the back increases.
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When this water evaporates into the air it
removes energy from the bulb as heat —
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you feel this effect every time you step out of the shower, the evaporating water withdraws
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energy as heat and chills you.
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This evaporation, this withdrawal of heat,
lowers the temperature and begins the condensation
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of the vapor, which starts the cycle as I
showed you with the cool, compressed gas.
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As long as the head is wet and heat is withdrawn
from it, the bird will always “drink,”
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but if you were to operate the bird in humid
air it would slow down because little water
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would evaporate, and if the air were at 100%
humidity the bird would stop because no water
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would evaporate at all.
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Now, to make this dramatic condensation happen
when the temperature is lowered just slightly
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— the evaporating water lowers the temperature
by only about three-tenths of a degree — the
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bird’s designer choose a highly volatile liquid.
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This means one whose boiling point is near
ambient temperature because for small changes
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in temperature there is a large change from
vapor to liquid and so the variation of pressure
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is large.
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Watch what happens as I “heat” the base
of the bird with my hand.
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You see the liquid level in the base dropping:
that’s because energy from my hand is converting
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some of the liquid into vapor, which increases
the pressure in this region . . .
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and that causes the liquid to rise to the head.
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Eventually I heat the vapor so much
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that it shoots up the stem.
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Now watch as I place
my hand around the head.
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Heat from my hand converts liquid to vapor,
which increases the pressure and forces the
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liquid back to the base.
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To test this explanation of the bird’s operation,
let’s activate the bird in different ways.
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As I noted it is the temperature difference
between its top and bottom that drives liquid
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to rise to the head.
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So, let’s see what happens if I point a
light at the base of the bird, which
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I’ve painted black so it will absorb the energy from the light better.
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As I heat the base of the bird, the liquid
rises, as before but …
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the bird tips backwards.
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The wet nose tilted the center of gravity
… and so I added some modelling clay to
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the nose to get the bird to tilt forward.
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And now when I turn on the light the liquid
rises, the birds drinks just as if there were
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liquid in front of it until . . .
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I turn the light off …
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and the bird drinks for a little bit longer
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until eventually . . .
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it comes to rest.
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Next, let’s see what happens if we use this:
Whiskey.
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Again, thoroughly wet the bird’s beak with
the liquid . . .
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stand it upright … and then we see again
the liquid rising in the bird
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. . . and then
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… it drinks.
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We can also now understand why the bird’s
rate of drinking differs among the three methods
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I used to “activate” the bird: a heat
lamp, whiskey and water.
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Roughly, the heat bird takes three drinks
for every one of the water bird,
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the whiskey bird takes two for every drink of the water
bird.
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The reason the bird drinks whiskey faster
than water is because the rate of evaporation
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of the alcohol is greater than that of water.
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This means that heat is withdrawn faster from
the head and so more vapor condenses in a
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shorter amount of time, which accelerates
the pressure difference.
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The heat lamp causes the greatest difference
of all, which highlights how an engineer thinks
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about this bird.
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To an engineer this bird is a heat engine.
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A heat engine turns heat differences into
work — mechanical motion.
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To see that recall that when the bird is just
about to drink that its head is at a lower
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temperature than its base, which is at ambient
temperature.
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Then when it “drinks” the pressure in
the head and base start to equalize so liquid
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returns to the base, but the overall temperature
of the bird is now
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just a little below ambient temperature.
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When it return to upright the base draws in
energy as heat . . . the head then rejects
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some energy as heat and the bird drinks again.
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These two flows define a heat engine: a device,
operating in a cycle that absorbs heat from
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a high temperature reservoir, converts part
of it into work, and rejects the remainder
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into a low temperature reservoir.
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The fact that this is a heat engine means
it’s related to the great machines that
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make our globalized world happen: among those
the mighty steam turbine that generates electricity,
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the giant diesel engine that propels container
ships across the oceans, and the great gas
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turbine that flies us around the globe.
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I’m Bill Hammack, the engineer guy.
