How Cathode Ray Tube TVs Work

Photo by Zayd Hashimoto.

Cathode Ray Tubes were the display technology for around seventy years. Their rule spanned from 1934, when Telefunken released the first commercial TV set (Wikipedia), to the year when CRT display sales became the minority: 2008 (The Verge). That is a dynasty.

Now, many people look down on this technology. They view it as an old and obsolete technology. They do not appreciate their technological heritage. Many people’s parents and grandparents watched CRTs for all their lives. And to them, it certainly was amazing.


Historical Background

Cathode ray tubes need to have a vacuum inside them; there need to be very small amounts of air molecules within. So, before this cathode ray technology could be discovered, there had to be a way to pump most of the air out of a container. This happened in 1855, when Heinrich Geissler invented the Geissler tube.

The Geissler tube was the first type of vacuum tube to have an extremely low pressure within, a vacuum so high that interesting phenomena, such as cathode rays, could be observed. Most of the air was pumped out of it by some sort of primitive vacuum pump; it is not clear which type. Most of the vacuum pumps of the day were pneumatic devices. But these could not attain a low enough vacuum pressure to make a Geissler tube. Most sources say that Geissler invented some sort of mercury vacuum pump to evacuate his tubes (possibly hand cranked, but probably not). But, there is not much information on which specific apparatus he used.


Around the time of the invention of the Geissler tube, in 1865, the Sprengel pump was invented by Hermann Sprengel. The Sprengel pump was able to create the highest vacuum achievable at the time. So, it is plausible that Geissler tubes were made in later years with this pump or that Giessler invented a similar version of the pump.

In more detail, a Sprengel pump is a type of primitive vacuum pump that took advantage of the properties of mercury to evacuate a chamber, usually a glass vessel, of air. Mercury is a very dense and heavy metal that is a liquid at room temperature. In a Sprengel pump, a reservoir of mercury is attached to a very narrow capillary tube. The reservoir of Mercury is sealed off from the outside world. Nothing can enter. Only mercury can leave. At the top of the capillary tube, a big glass chamber, to hold the vacuum, is connected. The whole system is sealed. It is completely closed off from air entering.

Mercury is fed at a slow drip into a chamber at the top of the capillary tube. This is the chamber that the vacuum chamber is directly attached to. When the mercury drips down into the tube, a tiny bit of air from the chamber is trapped under the mercury and is pushed down by the heavy metallic liquid, partly because of the funnel shape of the glass transition from the chamber to the thin capillary. This trapped air is released at the end of the capillary tube when the mercury drips down into an open container. So, since air is being removed from this closed system, a vacuum is created in the attached vacuum chamber. An extremely low pressure can formed after waiting many hours, around 1×10−8 times the pressure of the atmosphere. A blowtorch can be used to melt closed (seal) and detach the vacuum chamber.


Sprengel Pump. R is the vacuum chamber. Image by Vladsinger, distributed under the CC BY-SA 4.0 license.

Mercury pump technologies are what enabled Geissler to make his high vacuum tube. Without the ability to make a high vacuum, the Geissler tube and subsequent vacuum tubes would have been impossible to make. The Geissler tube was special because it was the first time that a very low air pressure, near a vacuum, was reached in a glass chamber. It was a magnificent achievement, both because of the fact that this tube was the first human made tube to be almost completely emptied of air, and because of the further scientific discovery and technology that it enabled. Usually, Geissler tubes were not completely emptied of gas. Geissler would remove most of the air from the tube and then introduce gasses such as Argon, Neon, Mercury Vapor, and so on. When a high potential, voltage, was applied across the tube, the gasses would fluoresce. In other words, the electrical energy in the gas would be transformed and released in the form of photons, light. This florescence is because noble gasses tend to fluoresce when their atoms are excited by electricity. Mercury vapor is not a noble gas, but it does share properties with the noble gasses, such as the property of florescence, due its electronic configuration. Mercury atoms are strongly resistivity of the removal of an electron. This effect caused Geissler tubes filled with these special gasses to glow in a pleasing way. Geissler tubes were mass produced just because of these property. Geissler tubes were not just the indirect precursors of the CRT: they were also the ancestor of the neon light.

So, how did the world get from Geissler’s tube to Televisions? Well, the next step in the journey was the Crookes tube. William Crookes, who was a chemist from Great Britain, like to observe and play around with Geissler tubes. One day, he was observing the distribution of florescence in the rarefied gas within the tube. Of course, the tube was electrified with thousands of volts. He noticed that as the tube was pumped down more and even more air was removed, a dark spot of apparent empty space formed near the cathode of the tube.

Right now is a good time to explain the terms cathode and anode, since these two parts of a discharge tube are very relevant to CRT televisions and the principles behind their operation.


Cathodes and Anodes

If you are an electrical engineer, this might be a little unnecessary for you. But, I am going to assume that the reader is relatively inexperienced in the field, and thus I will provide a more thorough explanation.

Have you ever seen a AA battery? Well, if you have, you have probably notice that it has two ends, two electrodes, one on either side of the battery. One of these side is the positive side (labeled ‘+’) and one of them is the negative side (labeled ‘-‘). The cathode is just the negative side and an anode is the positive side of some material, object, or circuit that has a potential difference across it. There are two types of electrical current flow: Direct current (DC) and Alternating current (AC). If AC current is flowing through a circuit, electrons alternate between flowing from the positive side to the negative side and flowing from the negative side to the positive. But DC electricity flows from the negative electrode to the positive one. So, electrons flow out of the cathode.

Many circuits, such as the mains electricity in your house, are energized with alternating current electricity. Mains electricity does one cycle of switching between positive to negative and negative to positive sixty times every second. But high voltage vacuum tube applications, such as Geissler tubes and CRTs, use direct current.


Crookes’ Tube

William Crookes created the Crookes’ Tube, a modified Geissler tube. Basically, Crookes removed from the production process the partial filling of the tube with noble gasses or mercury vapors because an aesthetic glowing effect was certainly not needed. The air in the tube, left over when the vacuum was not too high, would still turn to plasma, just as strong as the amount of air left in the tube. Crookes ran experiments with his modified tube by observing the plasma formed when a potential difference of thousands of volts was applied across the tube (by the way, Crookes discovered the ionized form of matter known as plasma). When the tube was only pumped down to a mediocre level, Crookes noticed that there was a small dark area near the cathode, the point where electrons flowed out into the near vacuum, contrasting the glowing plasma near the anode. As Crookes pumped more and more air out of the tube, the dark area enlarged until it extended all the way to the anode. This meant that the electrons were forming an invisible beam, travelling through the vacuum. This was the electron beam that is integral to the operation of a CRT TV.

Cathode Ray Tube

While experimenting with the Crookes tube, Crookes noticed that when a phosphor was placed at the end of the tube, opposite the cathode but next to the anode, the target of the electron beam, phosphorescence would occur. ‘What is phosphorescence?’, you might ask. Phosphorescence is when atoms, a substance, are excited by energy. In this case, the energy is provided by the free electron beam. Then, the atoms’s electrons go back to the lower energy state. This releases light. The difference between phosphorescence and florescence is that phosphorescence lasts for a little while, up to a few seconds after excitation, while florescence ends immediately. The effect of this phosphorescence when hit by a free electron beam was that a bright spot of light on the phosphor would appear when the electron beam was turned on. Crookes’ Tubes from that point on were made with the interesting phosphor at the end.

Then, in 1897, Karl Ferdinand Braun invented the CRT oscilloscope when he made a machine containing a so called Braun tube, a descendant of the Crookes tube. This oscilloscope showed a picture on a phosphor tube using similar technology to the CRT (but there was some electrostatic deflection in the place of magnetic deflection). Then, in 1929, Vladimir Kosma Zworykin invented a type of CRT called the Kinescope. This was a primitive television tube that could receive and display a picture or actually act as a video camera. Finally, in 1931, Allen B. Du Mont invented the first practical and durable television CRT that could actually be sold as a TV.

So, let’s look at a diagram of a CRT to finish the explanation, and then wrap things up.

Image of a CRT by
wnr (homewiki), distributed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

This is an advanced color television CRT. Of course, there is a high vacuum inside the whole tube. At 1 are the three electron guns, one for the three beams. A tungsten filament heats up an electrode, the cathode, and this causes thermionic emission of free electrons to occur. This makes a more intense electron beam than would be possible with Crookes’ tube. At 2, you can see the three electron beams. They have no color, but each of them is assigned to hit a specific color phosphor dot. There are three phosphor dots: red, green, and blue. When the intensity of the beam hitting each of them is varied, every color in the world can be produced. Near the place where 2 is located would be the first, second, and even third anodes. These have increasingly higher voltage and help to accelerate the electrons. 3 are the focusing coils. These electromagnetic coils exert a magnetic force on the charge electrons and cause their beams to focus until they are pencil thin. At 4 are the two sets of deflection coils: horizontal and vertical. These electromagnetic coils can bend the beam across the coordinate grid that is the screen and cause the beam to strike anywhere. At 5 is the final anode cap (known as the “ultor”). This is usually at around 30 thousand volts DC. The electricity is generated by a Cockcroft-Walton multiplier fed by a flyback transformer. The flyback emits high voltage saw tooth AC from DC (or AC) input. The Cockcroft-Walton takes in the sawtooth AC and converts it into even high voltage DC. This final anode cap is the place where the high voltage DC is fed into the vacuum chamber and is connected an internal electrical coating which is energized and becomes a balanced anode (the beam cannot be skewed). At 6 is the shadow mask which separates the beams destined to be either red, green, or blue. This makes a crisp picture and crisp colors. At 7 is the phosphor screen that, when hit by the beam, lights up. This is a quick-decay phosphor, that only holds light for one cycle. The beam scans across the entire screen in rows from top to bottom sixty times per second. This creates an apparent perfect picture due to the phosphorescence decay delay and the fact that human eyes cannot perceive really fast changes. 8 is a close-up of the phosphor screen that shows the red, green, and blue phosphors. These phosphors emit mostly visible light in their namesake frequency, but a tiny bit of UV light is also released. In front of the phosphor screen is a really, really thick (1 inch sometimes) leaded glass screen to prevent X-rays from being released. So, that is the cathode ray tube. Most of the circuitry in a CRT goes to powering the tube, the coils, and the signal amplifiers. CRT Televisions may seem kind of simple, but they are very advanced.


So, next time you see a CRT TV by the side of the road, or at the dump, do not dismiss it as obsolete junk. Try to look past the clunky exterior and see the advanced and revolutionary technologies that are contained within. Think of how the technology lying on the side of the street, abandoned and exposed to the elements, changed the world and the way we lived. Think of the giant vacuum tube, high voltage, and electron gun contained within, and just think how impressive it is that mere mortals such as ourselves were able to harness the laws of physics to entertain ourselves. A CRT is more than a Television: it is an integral element of human history that deserves to be remembered and cherished, just like cave paintings on the wall or ruins in Athens.

Works Cited:,conditions%20for%20temperature%20and%20pressure.&text=Mercury%20has%20a%20unique%20electronic,similarly%20to%20noble%20gas%20elements.,acts%20as%20an%20electron%20donor.,direction%20of%20the%20electron%20flow.),by%20a%20beam%20of%20electrons.

Leave a Reply

Skip to content