- A man named JJ Thompson discovered the electron in 1897
and there are tons of videos about it
but most of them ignore what Thompson himself
said was the motivating factor.
A debate about how cathode rays move.
Want to know not only how
but why electrons were discovered.
Let's go.
♪ Electricity ♪
- First a little background about Joseph John Thompson,
JJ on publications, to friends, and even to his own son.
Thompson was born in Lancashire England
to a middle-class book seller.
When Thompson was 14, he signed up to be an apprentice
to a locomotive engineer.
However, they had a long waiting list
and to fill the time he applied to and was accepted
as a freshmen at Owen's college.
Thompson later recalled that, quote,
"The authorities at Owen's college
thought my admission was such a scandal.
I expect they feared that students would soon be coming
in strollers, that they passed regulations
raising the minimum age for admission
so that such a catastrophe should not happen again."
While in school Thompson's father died
and they didn't have enough money
for him to get the apprenticeship.
Instead he relied on scholarships at university,
ironically leading him to greater fame.
In 1884 at the tender age of 28, Thompson applied
to be the head of the Cavendish Research Institute.
He mostly applied as a lark
and was surprised as anyone to get the position.
He said, quote, "I feel like a fishermen
who has cast the line in an unlikely spot
and hooked a fish much too heavy for him to land."
Suddenly Thompson had a lot of resources, stability
and ability to research whatever he wanted.
He ended up having an uncanny ability
to find new material to research.
In fact, a full eight of his research assistants
and his son eventually earned Nobel prizes.
But of course, like Thompson's own Nobel prize
that was in the future.
Why did JJ Thompson discover the electron in 1897?
Well, according to his autobiography
quote, "The discovery of the electron
began with an attempt to explain the discrepancy
between the behavior of cathode rays
under magnetic and electric forces."
What did he mean by that?
Well, a cathode ray or a ray in a vacuum tube
that emanates from the negative electrode
can be easily moved with a magnet.
This fact gave a charismatic English chemist
named William Crooks, the crazy idea that the cathode ray
was made of charged particles in 1879,
however, five years later a young German scientist
named Heinrich Hertz found that he could not get the beam
to move with parallel plates or with an electric field.
Hertz decided that Crooks was wrong.
If the cathode ray was a beam of charged particles
then it should be attracted to a positive plate
and repelled from the negative plate
and Hertz decided it was some kind
of new electromagnetic wave,
like a new kind of ultraviolet light.
Further, in 1892 Hertz accidentally discovered
that cathode rays could tunnel through thin pieces of metal
which seemed like further proof
that Crooks was so very wrong.
Then in December of 1895, a French physicist
named Jean Perrin used a magnet to direct the cathode rays
into an out of an electroscope called a Faraday cylinder
and measured its charge.
Perrin wrote quote, "The Faraday cylinder
became negatively charged when the cathode rays entered it
and only when they entered it.
The cathode rays are thus charged
with negative electricity."
This is why JJ Thompson was so confused.
He felt that Perrin had quote "Conclusive evidence
that the rays carried a charge of negative electricity."
Except that quote, "Hertz had found
that when they were exposed to an electric force,
they were not deflected at all."
What was going on?
Thompson wondered if there might've been something wrong
with Hertz's experiment with the two plates.
Thompson knew that the cathode ray tubes that they had
only worked if there's a little bit of air in the tube
and Thompson wondered if the air affected the results.
Through trial and error Thompson found
he could get a stronger beam by shooting it
through a positive anode with a hole in it.
With this system he could evacuate the tube
to a much higher degree.
And if the vacuum was good enough the cathode ray
was moved by electrically charged plate
just as negatively electrified particles would be.
If you're wondering why the little bit of air affected it
it's because the air became ionized
in the high electric field and became conductive.
The conductive air then acted like a Faraday cage
shielding the beams from the electric field.
As stated before Heinrich Hertz also found
that the cathode ray could move through thin metal.
How could a particle do that?
Well, Thompson thought maybe particles could do that
if they're moving really, really fast.
But how to find how fast the cathode beam was moving.
He used the fact that he could move it
both with an electric field and a magnetic field
to make an electromagnetic gauntlet.
First Thompson put a magnet near the ray
to deflect the ray one way, and then he added charge
to the plate so that the forces were bound
and the ray went in a straight line.
Thompson knew that the force from the magnet
depended on the charge of the particle, its speed,
and the magnetic field, given the letter B.
He also knew that the electric force from the plates
only depended on the charge of the particle
and the electric field.
Since these forces were balanced
Thompson could determine the speed of the particle
from the ratio of the two fields.
Thompson found speeds up to 60,000 miles per hour,
or up to a third of the speed of light.
Thompson recalled quote, "In all cases
where the cathode rays are produced
their velocity is much greater than the velocity
of any other moving body with which we are acquainted.
Thompson then did something even more ingenious.
He removed the magnetic field.
Now he had a beam of particles moving at a known speed
with a single force on them.
They would fall, as Thompson said, quote,
"Like a bullet projected horizontally with velocity V."
Note that the gravitational force
is much, much smaller than the electric force.
And so it's light objects and therefore can be ignored.
By measuring the distance the cathode ray went,
he could determine the time they were in the tube
and by the distance they fell
Thomson could determine their acceleration.
Using F equals ma, Thompson determined the ratio
of the charge on the particle to the mass or E over M.
He found some very interesting results.
First, no matter what he changed about the experiment
the value of E over M remained constant.
Quote, "We may use any kind of substance we please
for the electrodes and fill the tube with gas of any kind.
And yet the value of E over M will remain the same."
This was revolutionary.
Thompson concluded that everything was filled
with these tiny little things
that he called corpuscles, but we now call electrons.
And that the corpuscles in one item
are exactly the same as the corpuscles in another.
So for example, an oxygen molecule contains
the same kind of electrons as a piece of gold.
Atoms are the building blocks of matter.
But inside the atoms, there are these tiny electrons
that are the same for everything.
The other crazy result he found
was the value of E over M was enormous.
About 1700 times bigger than that for a charged
hydrogen atom, which had the biggest value of E over M
for anything they had found before.
Which meant that either these corpuscles
were very, very highly charged, or they were tiny.
A student of Thompson's named CTR Wilson
had experimented with slowly falling water droplets,
and found that the charge on the corpuscles
were to the accuracy of the experiment,
the same as the charge on a charged hydrogen atom.
Thompson therefore came to some
really interesting conclusions.
One, electrons are in everything and are well
over a thousand times smaller than even the smallest atom.
Two, Benjamin Franklin thought positive objects
had too much electrical fire, and negative had too little.
Really positive objects have too few electrons
and negative have too many. Oops!
Three, although since Franklin people thought
current flowed from positive to negative,
really the electrons are flowing the other way.
When a person talks about the current
that flows from positive to negative,
they're talking about something that's not real.
True electrical current flows from negative to positive
and that's the way the electrons move.
However, by the time people believed JJ Thompson
it was too late to change our electronics.
So people just decide to stick with quote,
"conventional current" going the wrong way. Sorry!
Four, since electrons are tiny and in everything
but most things have a neutral charge
and because solid objects are solid
the electrons must be swimming in a sea or soup
of mildly positive material like raisins in a raisin cookie.
The first three conclusions are still considered correct
over a hundred years later.
The fourth theory is called the Plum Pudding Model
and it's named after a truly English dessert
that they tortured people with during Christmas,
was proposed by Thompson in 1904.
In 1909, a former student of Thompson's,
named Ernest Rutherford was experimenting with radiation
and inadvertently demolished
the Plum Pudding Model in the process.
However, before I can get to Thompson's gold foil experiment
I first want to talk about what was happening in France
concurrent to Thompson's experiments.
This is the story of how a new mother
working mostly in a converted shed
discovered and named the radium and radiation
that Rutherford was experimenting with.
That woman's name was Marie Sklodowska-Curie
otherwise known as Madame Curie.
And her story is next time on the Lightning Tamers.
That's my video, please remember to subscribe
please remember to give it a thumbs up.
Also, if you're confused about the history
of the cathode ray, I have videos about that.
If you're interested in more about Heinrich Hertz,
I have videos about that.
I have a lot of videos.
Why don't you subscribe?
And then you can spend all day watching my videos.
Okay. Have a nice day.
Please remember to put a comment, it always makes me happy,
unless your comment is I didn't like your video.
And please remember to give it a thumbs up
unless you're planning on giving it a thumbs down
in which case skip it.