CRT and Vacuum Tubes
Why do they call a vacuum tube a Cathode-Ray Tube? What
is a cathode ray?
Inside a vacuum tube there is a cathode that has a negative charge (extra
electrons) and an anode that has a positive charge (that would like to take
more electrons). To help the electrons move from the cathode to the anode
we heat the cathode. (That is why a vacuum tube had a heating filament.)
The flow of electrons from the cathode is called a cathode ray. We
sometimes talk about an "electron gun." We can control where the direction
that the gun fires with charged plates along side the cathode ray. A
picture tube (like a TV or a computer monitor) works by having the electron
gun shoot at a screen that glows when the electrons strike it. This cathode
ray gives the cathode ray tube its name.
The flow of electrons is one-way and can be controlled by placing a grid
with a varying charge between the cathode and anode. These two
characteristics allow us to use vacuum tubes as diodes and amplifiers also.
Physicists in the 19th century studied electrical phenomena that occurred in
long glass tubes in which the air (or most of the air) was removed. They
discovered that some sort of rays or particles were emitted from a hot
"cathode" when there was a negative voltage applied. Typically the
"cathode" was a thin piece of wire at one end of the tube. The rays were
always negative, not positive, and would travel from the cathode to the
"anode." The rays were identified as electrons. You can find pictures of
these tubes on the Internet and in books.
Actually, for decades there was confusion because there was still small
amounts of air in the tubes, which confused the measurements because the air
molecules would become ionized. But they finally got it figured out.
Physicists found that the electrons could make a phosphor (or even plain
glass glow when the electrons had had a voltage of several thousand volts.
This was very useful.
Why didn't the physicists call their device an "electron beam tube?" They
certainly could, but they were following the nomenclature of chemists
who had been doing chemistry for hundreds of years before them.
When chemists made an electrolytic cell, the negative side of the cell was
called a cathode. It seemed natural for the physicists to also call the
negative part of their tube the "cathode" because at first they did not know
that the rays were really electrons.
Also, back then, people were discovering "rays" of all kinds. So the name
"cathode ray" tube stuck. Nowadays we might call it an "electron beam" tube
because that is exactly what it is.
In my research lab we often buy "electron beam guns" to put in our vacuum
chambers. These "guns" emit a beam of electrons for various purposes.
Early vacuum tubes had only 2 electrical surfaces inside them. The one
meant to be attached to the positive side of an electrical source is the
"Anode" The surface connected to the Negative source is the "Cathode".
When electricity flows, it is electrons moving from the negative source to
the positive. Inside a vacuum tube, these electrons can actually become
visible. These 'rays' of electrons that were observed coming off the
cathode inherited the name "Cathode Rays".
A Cathode ray tube is a special type of vacuum tube that uses these flowing
electrons for another purpose. Because electrons can be attracted to
magnets or electrical sources, it is possible to steer a tight beam of them
to strike a certain point on the opposite side of the tube. (like the back
of a TV screen.) When that surface is covered in a phosphorescent coating,
it glows when the electrons strike it. Lastly, by controlling the intensity
of the stream of electrons (Cathode rays) you can control how brightly the
screen will glow at that particular point. In a TV, the stream moves from
left to right, and top to bottom on the screen, like a book. It completes
30 scans per second, but the tendency of the phosphorescent material to glow
for just a moment afterwards keeps the screen from flickering.
A radio vacuum tube is sometimes called a cathode-ray tube because it has
several terminals in it which can emit and/or collect electrons. The
cathode emits electrons. In partially evacuated tubes with a neon gas
filling, for example, these electrons can ionize the gas which produces
light along the path of the electrons. This light, produced when the
ionized atoms return to their ground state by emitting photons were called
cathode rays in the early days before they were well understood.
Incidentally, the terminal that collects the electrons in a vacuum tube is
called the anode.
Best, Dick Plano...
This is a historical thing. The part of a vacuum tube which is heated,
to "boil off", or emit electrons, is called the cathode. The electrons
fly across the vacuum tube, perhaps modulated by a grid or two, and are
then collected by an "anode". These same words show up in chemistry,
though for a battery things may reverse as far as names. But the tube
emits from its cathode. I do not know why people added the word "ray",
of course electrons are both particles and waves, but it could have been
called a cathode tube, a cathode electron tube etc. etc. All this dates
to about the 1920's. This tube has now become the same thing as your TV
tube, either in your computer monitor, or TV set. But again, this is
just language, as really all tubes are CRT's. But regular tubes have
gone away, for the most part, replaced by transistors, and integrated
circuits starting in the 1960's.
In the first days of discovering vacuum tubes,
people knew the electrodes were emitting something which tended to travel
in straight lines inside the evacuated bulb.
If a little gas remained in the tube, and some electrodes cast shadows,
they could see faintly glowing lines like sunbeams in fog.
If no gas remained in the tube, the glass on the far wall of the tube
but only in places with line-of-sight exposure to a particular electrode.
So these could be nonspecifically referred to as "rays".
The electrodes got names, the negative electrode was called the "cathode",
and the positive electrode was called the "anode".
Sorry, I have not heard how these names started.
Cathode rays are the rays emitted by the cathode (the negative electrode
of a vacuum tube).
At first nobody really knew what was happening inside a vacuum tube, or
what these rays were.
So the cautious name "cathode ray" was often used for a while.
A little later we learned that these rays are electrons freely coasting in
after being extracted from a solid by high voltage or heat.
Today, "Cathode Ray Tube" usually refers to the specific kind vacuum tube
we use for visual displays like TV.
Other vacuum tubes, though they use freed electrons,
depend a little less on exactly straight-line travel,
and are usually given other names such as "electron tubes".
Some vacuum tubes intentionally have a little gas in them
(neon lamps, fluorescent lamps, voltage-regulator tubes, krytrons &
They use positive ions as well as electrons,
and it could be considered technically incorrect to call them "cathode ray
"Plasma tubes" or "ionization tubes" might be better names.
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Update: June 2012