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Name: Jamie
Status: educator
Grade: N/A
Location: PA
Country: N/A
Date: 1/24/2005

Why do they call a vacuum tube a Cathode-Ray Tube? What is a cathode ray?

Jamie -

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.

Larry Krengel

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.

Bob Erck

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.

Ryan Belscamper

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.

Steve Ross

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 might glow, 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 the vacuum, 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 & ignitrons). They use positive ions as well as electrons, and it could be considered technically incorrect to call them "cathode ray tubes". "Plasma tubes" or "ionization tubes" might be better names.

Jim Swenson

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