funding for this program

was provided by
the annenberg/cpb project


in the 18th century,

electricity was 
just a curiosity.

but in the 19th century,

in became one of the most
important tools

not only of science
but of industry.

and the crucial invention
that caused that transformation

was made just in the year 1800
by alessandro volta.

and it was,of course,
the electric battery.

i wanted to bring in
a car battery to show you,

but when i went to
the automobile showroom,

all they had was a lemon.

so i thought
i'd make a battery

out of a lemon.

if we're going to use this
as a battery,

we need electrodes to
take the electricity out with.

so i'll just put in
a zinc nail like that.

and a copper nail like that.

to show you that
this really is a battery,

i'll connect it to
that galvanometer,

or digital voltmeter,
right over there.

now watch.

when i conect it,

you'll see that it will read
a little bit under one volt. 

the volt,incidentally,
was named after

alessandro giuseppe antonio
anastasio volta.

his full name is too long,
so they shortened it to volt.

the galvanometer was named
after giuseppe galvani

are the principal characters

in the story
that we want to tell today

it would be nice

to be able to do something more
with our lemon battery

than just connect
the voltmeter across it.

it would be nice to run
this electric motor with it

i can try to do that

by connecting the electric motor
to my battery like this.

but if i do that
and then close the switch.

absolutely nothing happens.

the reason was that

is that although the lemon
really is a battery,

it's not a very good battery

it wasn't designed
to be a battery

it was designed to be a lemon

that means that although
current runs out of the lemon

it dosen't run out fast enough
to run the motor.

it runs out very slowly.

there's a cure for that.

all i have to do

is store up the current
in a leyden jar,


or in this modern-day version
of the leyden jar,

in a capacitor.

what i'll do is disconnect
the motor from the battery

for the moment

and instead connect
the leyden jar,

or capacitor, to the battery.

as time gose on,
these will charge up,

and eventually the voltmeter

will reach the reading
that it had before

when that happens

i'll be able to run
the motor from the lemon

that will take a while,

so let's go on
with today's story,

which is how the electric
battery actually works.

the electric battery.

it's been around
since the 1800s

in many containers and under
a variety of circumstances.

but how does it actually work,


or not work,
as the case may be?

en route to the answer--

and this is true
with any hot topic in science--

part of the fun
is getting there

[car won't start]

to understand
how a battery really works,

one way to get started
is to make a metal from scrach.

beging with asngle positive ion--

that's an atom
missing just one electron

because it has
a positive charge,

it creates the potential
energy of attraction,

as shown here on the graph,
for the missing electron

then place more ions 
side by side

one next to another

add just enough electrons

to make an lectrically
neutral system.

then arrange then
in an orderly lattice

to create a piece of metal.

an imaginary test change

would detect no change
in potential enelgy

and therefore no force

anywhere inside
or outside the metal

is acted on by all the ions
and all the electron

expect for itself,

leaving itselfe
a net potential enelgy.

if that electron is moved

from one part of the metal to another

the othere electrons
flow around to replace it

so that it has a same
potential energy everywhere

that means
there's virtually no force

to prevent an electron

from moving freely
through the metal

but beyond the surface
of the metal

there are no more ions
and other electrons

to help balance forces

so,to push an electron
out of the metal

requiress a powerful force

creating a real
electric potenial

and leaving a net
positive charge on the metal

so the overall potential enelgy
of a real electron

drops sharply
at the metal's surface

to a lower value that's
the same everywhere inside

the electrons of a metal
also have kinetic energy

but not enough to escape

the amount of extra energy

an electron would need
to escape from a metal

is known as the work function

alessandro volta came along

before anyone was able to
construct an electric battery,

much less see
the inner workings of a metal.

but volta did have 
a knowledge of metals,

their properties,
and how they work 

and in that regard,

alessandro volta was
considerably ahead of his time

science was volta's gift.

and he wanted to share it
with the world.

in 1775, he wrote
to a friend in england
 
describing a recent invention

which he called elettroforo perpetuo

or"the perpetual reservoir 
of electricity"

it later became known as the
electrophorese

once charged

the electrophorese retained

an unlimited number
of induced charges

it was indeed a perpetual
reservoir of electricity

because the charges remauned

no matter how often 
the device was touched

the scientific community
hailed volta's achievement,

and in england

joseph priestley called it

the most significant invention
since the leyden jar

while the invention
of the electrophorese

gave science
an oppotunity to advance

it provided its inventor
the oppotunity to visit

some of the countories

he'd only known about
through correspondence.

in holland for example,

volta studied

martinus van marum's
famous electrical machine.

and in france, he was accepted
by the avant-garde

in both science and the arts

paris was the city of
pierre simon leplace,

astronomer and
mathematical physicist,

and the graet chemist
antoine lavoisier.

it was also
on a visit to paris

that the clonial
printer-turned-electrician,

benjamin franklin,

entertained volta at dinner.

volta arrived in paris in 1781.

and during his stay,


he accomplished much
in the way of science

in fact, in the words
of one parisian admirer,

volta was hailed as
"the newton of electricity"

and so it was only natural

that he visit
the english universities

before returning home to italy.

as a scientific celebrity

and professor of
experimental physics

at the university of pavia,

at the age of 46,

volta might have rested
on his well-deserved laurels.

however, volta happened to
receive a timely letter

from one luigi galvani.

galvani lived bologna,
and like volta,

he was an italian given to

bold ideas

and shocking discoveries.

perhaps the boldest
and most shocking 

was an idea galvani called
"animal electricity"

"this was vital force"
wpote galvani,

"different from
ordinary electricity

and it could be found
in the tissue of a frog."

then as now, an electric impulse
causes a frog's leg to twitch

in other words,

when the tight nerve
of a freshly dissected frog

is given 
a small electric impulse

a muscle contraction occurs

by observing the twitching
of a severed frog's leg,

galvani had discovered

that those nerve impluses
which excite muscles into action

are really electrical signals

and those signals, he said,

travel everywhere
throughout the body.

stored in the frog's nerve,

like ordnary elelctricity stored
in a leyden jar.

he thought that a discharge
of animal elelctricity

made the muscle move.

but volta viewed galvani7s
discovery quite differently.

rather than detecting
so-called animal elelctricity

in all parts of the body

volta found the impluse
only in the frog's leg.

and,he reasoned,

that animal electricity
was ordinary electricity--

merely a jolt that was generated

when two metals
in galvani's circcuit

happened to come into contact.

even today

ther's a very significant question

at the heart
of volta's argument--

why is an electric impulse created

when one metal
touches another?

each metal--








zink,for example--

has a work function that keeps
its electrons from escaping.

But a different metal--
say,copper--

has a different work function.

These differ by the equivalent
of about one volt

of electric potential.

When two metals
are brought into contact, 

the barrier
at the interface vanishes,

and electrons are free
to flow into the metal

where they have 
lower total energy.

As electrons flow,

the metal they leave
becomes positivery charged,

and the one they enter
becomes negativery charged,

creating an electrostatic
potential difference

that finally balances
the energy difference.

then the flow stops.

If the two metals 
are now separated,

each has 
a net electric change,

and an electrostatic potential
difference exists between them.

Of cource,

since the eletron hady't been
discovered in volta's day,

he'd have to base his argument 
on different grounds.

So,he invented 
the voltaic pile,

which resembled
an electric eel.

His intention may have been 
to shock calvani.

But in any event,

one reasen the eel
produces electricity

is because a certain amount
of moisture

flows between its fin dicks

and acts as an electrolyte,

which is a fluid containing

positive and negative ions
in solution.

Suppose two different metals,

electricarry charged
by contact,

are placed in an electrolyte.

The metal that has 
extra electrons 

attracts positive ions
from the solution.

Where ever positive ions
touch the surface,

they can extract excess
electrons from the metal.

Meanwhile,the other metal,
which lacks electrons,

attracts negative ions.

When negative ions 
reach the sureface,

the missing electrons 
can be replaced.

So,the electrolyte
tends to drive the metals

back to an electricarry
neutral condition.

If they were to make 
direct contact again,

a new surge of electrons
would flow

because of their difference
in work function.

But now,
because of the electrolyte,

the surge does't stop.

Electrons keep flowing
from one metal to the other.

And they're 
continuously replaced

by the ions in the solution

until all the chemical energy
in the solution

has been used up.

But until that happens,

a battery sheds light
on both physics and chemistry.

Chemical reactions

at the interface between
the electrolyte and one metal

provide extra electrons
ready to flow.

At the point where one metal
contacts another,

the difference in energy
of electrons inside each

drives electrons along...

Through the electric circuit
and back into the solution...

Where they complete
the chemical reaction.

And so it gose--

chemical energy
turning into electrical energy

until the chemicals are used up
and the battery is dead.

That's the theory
behind volta's invention,

ana to a certain extent,

the reason the electric battery
and the electric eel

work the same way.

So to duplicate nature

and prove galvani wrong
about animal electricity,

volta built layers of ‡€fins‡€

that were separated by
a kind of electrolyte--

in this case,alternating plates
of silver and zick

separated by moistend
pasteboard.

As with the eel,

here,too,it was the moisture
that did the trick--

the trick of acting
as an electrolyte.

And as with the electrophorese,

this invwntion was hailed
as a tremendous success.

For example,napoleon bonaparte,

an emperor always interested
in current events,

hailed volta's invention
as the greatest of all time.

And while that's strictry
a matter of opinion,

there's no question
that volta had developed

the world's first
electric battery.

Past or present,

two pieces of different metal
in an electrolytic solution

are the makings
of an electrical battery.

However,to make an efficient,
practical battery,

a little more is needed.

Things have come a long way

from the days when volta made 
stacks of different metals

separated by
pieces of moist cloth,

clay,wood,and skin.

Today,for example,
furnaces melt heavy lead bars.

Then,the lead is cast
into grids or plates

that will serve as electrodes.

Here,the plates are coated
with a mud paste--

a mixture of lead oxide,
acid and water.

Filler is added 
to the batch of mud

that gose on 
the negative plates.

Then the plates are assembled,

alternating positive 
and negative

with a fiberglass mat
in between each pair.

The plates are welded to contacts...

Placed inside a plastic case,
and secured.

The cases are filled with
a solution of sulfuric acid.

And finaly,
inside their case,

the lead plates
and the surrounding electrolyte

are transformed,at last,
into electric batteries.

But if both electrodes
are made of the same material--

that is,
if both electrodes are lead

rather than,say,
zick and copper--

how can these batteries work?

The answer was provided

by a french scientist
named raimond plante,

who discovered this new way
of making a battery.

He put two electrodes
of the same metal,lead,

into an electrolyte,

a weak solution 
of sulfuric acid.

Then he applied a voltage
between the electrodes,

driving electric current
through the solution.

This had the effect
of oxidizing one electrode,

changing its work function.

After that,the principale's
the same as volta's pile,

but this battery
had a great advantage--

it was reversible.

Once charged,
it was ready for use.

Once used and discharged,
it could be charged up again.

That's the basic idea behind
the modern lead-acid battery--

the battery of the automobile.

Of cource,there's a lot more
to today's automobile

than a battery.

But that's just 
what it takes,nonetheless,

to get those complex
machines started.

So drivers around the world 

owe a great deal
to alessandro volta...

Whether they're blessed
with a cream puff

or cursed with a lemon.

Alessandro volta's father
was one of for brothers,

all four of whom were priests.

This was not very promising
for the future of the family.

So,after 11 years
in the priesthood,

he quit to propagate
the family name.

He obviously did 
a good job of it

because we're still using it.

He produced six children--

three priests,two nuns,
and alessandro.

This was a very effective
italian method of birth control.

Alessandro,however,
was not cut out

for the preestly calling.

A colleague of his 
named lichtenberg said of him

that alessandro
understood electricity--

the electrcity of women.

Speaking of electricity,
I think the time has come

that we can
run our electric motor

because we've gathered
enough charge

fron the lemon battery.

All I have to do
is to connect the motor

to the capacitors like this,

and when I throw the switch,
we should have it.

Here we go.

And there it is,
lemon power.

Well,there's just no way
I'll ever top that,

so we'll take this up again
next time.

Captioning is made possitible by
the annenberg/cpb project

captioning performed by
the national naptioning
institute,inc.

Captions copyright 1986
california institite
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 annenberg/cpb project

the mechanical universe

is a college cource

with textbooks published
by cambridge univercity press.

For more information
about the cource,

video cassettes,
off-air videotaping,

and books based 
on the series,call...

End of file*****