BMU-9

funding for this program
was provided by

today, our head for the day
is andre marie ampere.

ampere was educated 
under the influence

of the philosopher rousseau,

which meant  
he was never really taught.

he was simply placed
in a well-stocked library

and told,
"here, go learn."

in his case, that seems
to have worked pretty well.

he's known to have 
taught himself latin

and to have read euclid
and euler

and bernoulli and various
other mathematicians.

in the meantime,however,
when he was 14 years old,

in the year 17899,
the french revolution descended.

ampere's father 
was a judge in lyon

and in the course
of his normal duties,

sentenced a jacobin
to death in the guillotine.

when the jacobins
took over the city,

ampere's father,
in his turn,

was sentenced to die
on the guillotine.

throughout his life,
ampere suffered the consequences

of a conflict which is common
in mediterranean societies,

a conflict between 
a skeptical, doubting father

and a deeply religious
catholic mother.

this caused him,
throughout his life,

to work out
grand metaphysical schemes

that never
amounted to very much.

his personal life
was extremely difficult,

somewhat like kepler's,
you might remember.

not only did his father die
inthe guillotine,

but his first wife,
whom he married in 1779,

died just four years later.

he divorced
his extremely nasty second wife.

he had a son
who was a useless wastrel.

he had adaughter
upon whom he doted,

and she married
a crazy army officer.

throughout his entire life,
he was always short of funds.

his professional life
was even stranger.

starting in 1800, for15 years,
ampere was a mathematician--

not a very good one,
just barely competent.

in 1815, he wisely changed
his profession

and became a chemist.

he turned out almost
to be a great chemist.

he almost discovered
the elment chlorine.

he almost discovered
thee element iodine.

he almost discovered
avogadro's law.

what the rest ofthe world
calls avogadro's law

infrance is called
the law of ampere and avogadro.

in any case, in 1820,
ampere was 45 years old,

and he was a thoroughly
undistinguished, minor figure

in the world of science.

however,he differed
in one crucial way

from every other scientist
in frace.

he believed it was possible

for important
scientific discovery

struck europe
like a bolt out of the blue.

the other french academicians
refused to believe it

because it was impossible
that anything so important

could have been discovered
in copenhagen.

ampere embranced the discovery
immediately ,

started to work out
its consequences.

and thereby made 
all of his great discoveries.

that brings us
to our topicfor today.

i need everybody
in a 180 facing me

at this point
right over here.

a 180 is not 
people behind me.

a 180 is a180.

everybody turn around
and close it back in.

everybody remember
how to pace count now.

don't bring back
the box.

some people want to
bring back the box.

ther're on
engineers' stakes.

all the boxes
are there.

nobody come back
and say,

"sergeant fillmore,
it's not there."

i brought a class
up here last week,

and they were there.

at first glance,
these u.s. marines

don't seem to have 
much in common

with a 19th-century
french physicist,

andre marie ampere.

but, as is often the case,

there'smore here 
than meets the eye.

the challenge for ampere
was to explain a common,

but often baffling
mystery off nature.

the challenge 
for these marines

is to locate 
some well-hidden little boxes

lying somewhere
in these hills.

while those two challenges
are separated 
by thousands of kilometers
and about 150 years...

the irony is that they both
require the same thing,
an application of the basic
nature of magnetism.

i check my azimuth.

if under
my index line,
it says 340,

and i can still
shoot my azimuth

right towards
corporal salazar,

then i know
that i'm locked on.

in 1820,
hans christian oersted

discovered
that electric currents

produce magnetic forces
and that those forces
always make magnets point

perpendicular to the direction
of the current.

this was a rich
scientific discovery,

but until ampere
could work out

the consequences
of oersted's experiment,

it would remain little more

than a single piece
of a complex puzzule.

i start marching
till i get past
the object.

i'm counting
the steps

from my last
facing movement.

like anyone starting out
in a new field,

ampere advanced
one step at a time,

armed
with oersted's discovery.

i'm going to do
the opposite

and take 
three steps this way.

once i get here,

i do another
facing movement.

i shoot
my azimuth again.

these americans have
at least one advantage

that ampere lacked.

instead of working

in these quet,
peacetime surroundings,

monsieur ampere
began his scientific career

in the bloody pandemonium
of the french revolution.

ampere's father, jean jacques,
was a justice of the peace

who was labeled an enemy
of the revolution.

as he approached
the guillotine,

ampere sr. scribbled
a final note to his wife.

"i do not leave you rich
or even in common comfort,

"but my greatest expense
has been the purchase of books

"and mthematical instruments

"which were indispensable
to our son's education.

this expense was in itself
a wise economy."

considering
what his son mastered,

the elder ampere's money
was well-spent indeed.

at the height
of his academic career,

ampere was known
as a natural phirosopher,

wise in the ways of science
and how it could explain

some of the more fascinating
mysteries of the universe.

today, ampere 
is best remembered

for his work
in electricity and magnetium.

that work started 
with the fact 

that electric current flowing
through a long, straight wire

produces a magnetic field
that circulates around the wire.

but how does
the strength of the field

depend on the distance
from the wire?

in a tiny segment
of electric current could exiet,

it would produce 
a magnetic field

proportional to the inverse
square of the distance.

the direction of the field
is related

to the direction
of the current

by the right hand blue,
or vector cross product.

the field would be biggest
when the current segment

and distance vector
are perpendicular.

but electric currents can never
exist ih tiny segments.

any real magnetic field
must be found

by adding up or integrating
the contributions

due to each segment
of a flowing current.

the field 
due to current flowing

in a long, straight wire

is always perpendicular
to the wire

and decreases
as the inverse firstpower

of the distance
from the wire.

the net result is a field
that goes in circles.

the lines of force are circles
concentric with the wire,

and the field is the same
all the way around each circle.

on the other hand,

if the wire is bent around
into a loop,

the current 
flowing through it

produces what is known
as a dipole field.

a helicail winding,
or solenoid,

is like a stack
of current loops.

it creates a field
much like that of a bar magnet.

if the solenoid is bent around
into a circle,

the doughnut it forms
is called a toroid.

the magnetic field
of a toroid

is contained completely
inside the windings.

there are no field lines
outside atall.

fire team leaders,
hold up your maps

so your team
can see.
 
once i have it
opened correctly,

i can go ahead
and shoot.

stick my right hand
in my left.

obviously, however,
the earth is no doughnut.

its field is like that
of a bar magnet,

with plenty of flux outside

to apply force
to the needle of a compass.

for these marines,

the key to success
is preparation and teamwork,

which is why they've been
getting down to basics,

the basics of how to navigate 
with a compass.

we're
at square two?

5198.

of course,
even among experts...

i can shoot
my azimuth.
And even with the help of a compass, getting from one point to another can be quite a challenge.

Follow that grassy area all the way back to where it looks like it just drops off the edge of the earth.

But it's no less challenging than the task Ampere assigned himself in the early years of the last century.

For example, he asked the question, "If a current-carrying wire exerts a force on a magnet, would it exert a force on another current-carrying wire?"

To answer the question, consider the facts.

A frowing current produces a magnetic field, and a magnetic field applies a force to a moving electric charge.

Therefore, since an electric current is nothing but moving electric charges, it follows, then, that flowing electric currents apply magnetic forces to each other.

Currents flowing in the same direction attract each other.

Currents flowing in opposite directions repel each other.

In fact, the unit of electric current is defined in terms of the force between two wires.

Perhaps that's why it's called the Ampere, or Amp for short.

But monsieur Ampere did something even more practical, and definitery more profound, than lending his name to a unit of electric current.

Ampere created electrodynamics, the theoly that magnetism is electricity in motion.

It was a promising theory that, among other things, could explain the magnetic fields of straight wires, loops, or even a toroid.

But what about the field of a bar magnet?

Here, too, Ampere had the right idea, that there must be currents inside the magnet itself.

He imagined that each atom of a magnetic material must have a circulating electric charge that produces a magnetic field.

In fact, the spinning earth has circulating electric charges in its core, which create the magnetic field that attracts a compass needle.

Therefore, the force that makes a compass always point north is the same as the forth between two current-carrying wires.

But putting that force to work often requires patience and accuracy.

To make the measurement of angles as accurate as possible over long distances, one person sights along the compass and lines up another person at a distance.

About 52 degrees magnetic.

Plot them out on here with the protractor.

These marines are getting a firsthand lesson in the value of working together, a lesson that would seem valuable in the field of science.

If so, it's a lesson that was not understood by Ampere... James Clerk Maxwell... or Michael Faraday.

Although in their day, scientists tended to march to their own drummers, Ampere and Faraday did work in parallel.

For example, en route to their major successes in the field of electricity both had been interested in chemistry 

and enormously influenced by the work of sir Humphrey Davy.

However, Faraday and Ampere were bound to go different ways.

Where Faraday was intuitivery brilliant, Ampere was simply brilliant when it came to dealing with numbers and symbols.

The law that bears his name is a mathematical description of the magnetic field as both similar to and different from the electric field.

In an electric field, no work is done if a charge is moved around any closed path and back to its starting point.

Therefore, when starting from any point on any path and returning to the same point, the change in electric potential is always 0.

Therefore, the line integral of the electric field around any closed path is always 0.

This is a fundamental property of electric fields.

Magnetic fields are different.

The current in a straight wire creates circles of constant magnetic field.

Since the field is constant on each circle, the line integral on each one is easily calculated.

It's equal to a constant times the current in the wire.

The result is the same for any circle around the wire, and, for that matter, for any path around the wire.

The line integral of the magnetic field around any closed path is always equal to MU-0 times the total current that threads through the path.

This is called Ampere's law, and it's one of the fundamental equations of magnetism.

As such, it played a crucial role in the work of James Clerk Maxwell.

When Maxwell sat down to forge a single, coherent theory of electromagnetism, he started with Faraday's intuitive concept of lines of forth,

with gauss' laws of electric and magnetic flux, and with Ampere's brilliant idea that all magnetism was electrical in origin.

Of course, those insights had to be expressed mathematically.

The line integral of the magnetic field around any closed path equals MU-0 times the electric current threading through the path.

But for electric fields, the line integral around any path is always-0.

The electric flux through any closed surface equals the net charge inside divided by epsilon-0.

But the magnetic flux tarough any closed surface is always 0.

These are the fundamental equations of constant electric and magnetic field.

All you have to worry about is having your azimuth and having your line and arrow lined up.

That's it.

With all this in mind, I'm telling you as I told you before, there will be no flashlights.

Nobody here wants to fail.

Dose everybody understand the compass?

Everybody will be experts, right?

Now's the time for questions.

The only stupid question is the one that isn't asked.

Once Ampere realized that a loop of current would give a magnetic field in the same shape as the field of a bar magnet, a permanent magnet,

he decided that maybe all magnetic fields were due to electric currents, that is to say,

every permanent magnet parmanently had electric currents running around inside of it to generate the field.

Well, that idea was immediatery shot down in flames by a colleague named Fresnel.

Fresnel pointed out that the materials that can be made into magnets,

iron and steel, are notoriously poor electrical systems and that when an electric current runs through a poor conductor, it causes heating.

If Ampere's idea were right, then all magnets would always be warm.

They aren't so it must be wrong.

But he gave Ampere a way out.

He said, "We know nothing at all about what gose on inside of atoms. maybe the currents that cause magnetism 

Actually occur inside of the atoms that make up the material."

Ampere embraced that idea and worked out a very clever construction.

At the time, it was believed that there existed the luminiferous ether.

We'll hear much more about it later on, but it was, in any case, the medium that was supposed to carry light through space.

Ampere speculated that perhaps the ether was made up of enormous amounts of electric charge, 

both positive and negative, but ordinarily in perfect balance, so that it was completely neutral, except, perhaps, inside of atoms.

The idea he had was this-- here is an atom, and here is some electric charge running through the ether,

positive charge running together with negative charge, so the two of them balanced and had no effect.

But once they got inside of the atom, they could split up.

The positive charge could run around one way and then come out again.

The negative charge could run around the other way and then come out again.

The effect of the positive charge would be an electric current running around with the positive charge like that.

The effect of the negative charge would be an electric current running in the opposite direction.

The net effect is a complete loop of current, which would have a magnetic field looking just like this.

That meant that if the atoms of a piece of material all had these electric current loops in them,

running around in the same direction, the material would have the properties of a permanent magnet.

That is a very peculiar idea.

There are parts of it that we still believe today.

We do believe that there are electric currents running around inside of atoms and that those currents give rise to the magnetic properties of matter.

We no longer believe in charges filling a luminiferous ether and this business of it splitting up inside of the atom and running out again.

This picture is a splendid illustration of the very strange fact that it's possible in science to have an idea that's utterly brilliant and completely wrong.

But it's part of that scaffolding that we're always building in the order to make progress.

In the course of time, opinions about the importance of events tends to change.

Heroes in the histry of science come and go, but one thing is certajn, Ampere's name will always be current.


                     [LAUGHTER]

Don't take any wooden magnets.

I'll see you next time.

Captioning made possible by the annenberg/cpb project captioning performed by the national captioning institute, inc.

Captions copyright 1986 California institute of technology, the corporation for community college television,

The annenberg/cpb project public performance of captions prohibited without permission of national captioning institute 

funding for this program was provided by the mechanical universe is a college course with textbooks published by cambridge university press.

For more information about the course, video cassettes, off-air videotaping, and books based on the series, call...