- On November 8th of 1895, a man named Wilhelm Roentgen
turned on a covered vacuum tube
and accidentally discovered the x-ray,
and changed our world.
This is not that story.
Instead, this is the story of why Roentgen
was playing with a covered tube in the first place.
Why would someone cover the tube?
What does that have to do with the creation
and the discovery of x-rays?
And how was the x-ray machine invented in the first place?
Ready for the story? Let's go!
♪ Electricity, electricity ♪
I'm gonna start in 1882.
That's when a 24-year-old German scientist
named Heinrich Hertz wrote his parents quote,
"I am busy from morn til night
with optical phenomenon in rarefied gases."
Hertz, like almost all of the scientific community,
was inspired by the experiments and conclusions
of a charismatic English chemist named William Crookes.
Four years earlier, Crookes demonstrated
that if he applied a high voltage
across a glass tube with a high vacuum,
or through a gas that was rarefied,
an invisible cathode ray would emanate
from the negative electrode,
and make glass on the other end glow.
He wasn't the first to do this,
but due to some clever experiments,
he was the first to think
that this was a ray of charged atoms.
This was a radical idea
and soon everyone was playing with Crookes tubes.
Hertz found, that although the cathode ray
would bend with a magnetic field,
it would not move with an electric field.
And Hertz decided that cathode rays
were a type of ultraviolet light,
concluding that quote, "cathode rays have no closer relation
to electricity than has the light produced
by an electric lamp."
By the way, Hertz didn't realize that trace amounts of gases
were insulating the cathode ray from the electric field.
And years later JJ Thompson figured that out,
and then demonstrate that these rays
are not ultraviolet light or charged atoms,
but actually charged subatomic particles called electron.
Hertz then moved from electrifying tubes
to electrifying an antenna which is how, in 1886,
he discovered the radio wave.
By the way, radio is an invisible electromagnetic wave
that has no closer relation to electricity
than the light produced by an electric lamp.
Crazy, huh?
In December of 1890, Hertz went to England
to receive the Rumford Medal for his work on radio waves.
While in England he met Crookes.
Perhaps for that reason,
Hertz went back to studying cathode rays.
This time he was interested in the power
and the heat from the cathode rays.
Now, in 1879, Crookes had melted a piece of platinum foil
by placing the foil in the middle of the tube
and focusing the cathode ray on it.
Hertz, however, used an unfocused cathode ray,
put the foil at the end of the tube against the glass,
and use gold as it easier to melt.
To his surprise, the gold didn't melt
and it also didn't block the path of the cathode rays.
He said that the gold leaf quote,
"looks like a faint veil upon the glass plate,
chiefly recognizable at its edges
and by the slight wrinkles in it."
This seemed to validate his theory
that cathode rays were really waves,
as he thought particles couldn't make it through a solid
without breaking it.
He was wrong.
Anyway, he tried it on a multitude of different metals
and found they all worked,
but he thought aluminum worked the best
because it was so easy to use.
Tragically, mere months after this publication,
Hertz started having severe headaches, quit researching.
He died on January 1st of 1894,
when he was just 36-years-old, from blood poisoning.
Before Hertz gave up on research,
he called his assistant Phillip Lenard and said quote,
"we ought to separate two chambers with aluminum leaf
and produce the rays as usual in one of the chambers,
and observed the rays in the other chamber."
Now Lenard had been fascinated with cathode rays
ever since he'd read about Crookes's experiments.
And also, years before, he'd read Hertz's old paper
that said that cathode rays were a form of ultraviolet light
and he had an ingenious idea.
Lenard knew that quartz crystals transmit ultraviolet light.
So Lenard made a tube with a quartz window
to let the cathode rays out of the tube and into the air.
Unfortunately for Lenard,
cathode rays are not ultraviolet light, so that didn't work.
However, after talking to Hertz,
Lenard now knew of an object
that would transmit cathode rays, aluminum.
Lenard recalled, he took out, quote, "the old tube, again,
replaced the quartz with a metal plate
containing a small hole sealed with aluminum foil,
spread a few grains of phosphor
on the small aluminum window, excited the tube,
and lo and behold, the grains glowed brightly."
The cathode rays had escaped the tube,
and even more surprisingly,
and Lenard said no one could have predicted this,
they not only could escape the tube,
they could travel a few inches in air.
Lenard was ecstatic.
It was quote, "a breakthrough into the unknown."
Lenard didn't know it, but he just made an x-ray machine
that was perfectly suited to discover the x-ray.
Years later, he complained quote,
"all Roentgen had to do was push a button,
since all the groundwork had been prepared by me."
Now, you should know that Lenard's tube
was actually not very good at making x-rays
and the Crookes tube, where he melted platinum
with a focused cathode ray, was a far better x-ray machine.
However, as Crookes was busy studying the platinum,
he never realized what he was creating.
He even accidentally developed
some of his photographic plates with his x-ray machine,
but he just thought the plates were defective
and return them to the manufacturer.
Over the years, several other people
accidentally took x-ray pictures with vacuum tubes.
Notably, Arthur Goodspeed in 1890,
Fernando Sanford in 1891, and Nikola Tesla in 1894.
Although none of them realized
exactly what they were looking at.
What made Lenard's tube so perfect
for discovering the x-ray,
is that it was the first one you study from the outside.
Inside the tube, it is very hard to distinguish
between the lights from the cathode ray
and from the lights from the x-rays.
Outside Lenard's tube, the only thing
that was supposed to emanate were cathode rays
through the aluminum window
and they only travel a couple of inches in air.
Therefore, all you needed to discover the x-ray
was a covered Lenard tube ina dark room, good vision,
an appropriate fluorescent screen,
and the luck to notice a glow a few feet away from the tube.
If you're wondering why Lenard didn't notice x-rays,
it was because he was using a fluorescent screen
made of light elements,
that was basically transparent to x-rays.
Although it worked fine on cathode rays.
And Roentgen happened to be using one
that was equally as efficient for cathode rays,
but about a hundred times more efficient
for detecting x-rays.
So how did Lenard's tube make x-rays
and why was the old Crookes tube
where he melted platinum, superior?
In both Lenard's and Crookes tubes
x-rays are made when a beam of fast moving electrons
hits a solid.
The quick moving electrons were created
when a high voltage was placed across the vacuum tube
and the negative electrons on or near the cathode
are repelled from the negative side
and speed towards the other side.
When electrons hit a solid,
they can create x-rays in two ways.
First, electrons can be diverted by the nucleus of the atom
and as they whip around the atom
they lose energy and produce x-ray.
This is called bremsstrahlung for breaking radiation.
If the atom is light, meaning having few protons
like aluminum, then it produces less breaking,
so it produces less breaking radiation.
But if the atom is heavy like platinum,
it creates more breaking radiation.
Second, the incoming electron
can actually knock out a low energy electron.
When this happens, an energy in a higher shell
falls down to fill in the empty spot
and releases that change in energy as an x-ray.
This is called x-ray fluorescence.
This produces more powerful x-rays.
However, with light elements these x-rays
are often not emitted,
as they tend to be reabsorbed by the same atom
to make it release another electron instead,
called the Auger electron.
In conclusion, aluminum makes less powerful
bremsstrahlung x-rays and almost no fluorescence x-rays.
This is why x-rays were first discovered
with a covered Lenard's tube,
but Crookes' platinum tubes
were quickly found to make more x-rays
and were used for x-ray machines
for the next 20 years or so.
It's easy to dismiss Lenard's contributions
because he turned into this big Nazi,
and pain in both Roentgen's and Einstein's side,
but he really did have an important role to play
in the history of x-rays and in physics in general.
This is not in any way to diminish what Roentgen accomplish.
For Roentgen didn't just notice x-rays,
but by himself, and with primitive equipment,
and over a course of just weeks, he found them, named them,
decided that they were electromagnetic waves,
that they came from the cathode ray hitting the wall,
that they were blocked by lead,
that their penetrating power
depended on the density of the material,
and even took the first medical x-ray of his wife's hand.
Once Roentgen took that first medical x-ray,
according to him, all hell broke loose.
And that story is next time on "The Lightning Tamers."
♪ Electricity, electricity ♪
Thanks for watching my video.
Please remember to give it a thumbs up.
If you haven't seen it already
and you're interested in how Hertz discovered radio waves,
I have a video about that.
And also make sure to check out the next one about Roentgen,
he's an interesting cat.
Okay, have a good day.
(upbeat music)