The Thermionic Tube Explained — Everything You Need to Know

Vacuum tube

The Thermionic Tube Explained — Everything You Need to Know

Key Points About the Thermionic Tube:

  • The Thermionic tube goes by many names, to include vacuum tubes, vacuum diodes, thermionic tubes, the Fleming valve, and kenotron. 
  • While the device has many names, the actual device only has three components. The device heats electrons, gives them away at one end of the tube, and receives them at the other end. 
  • Basically, the vacuum tube starts electrons moving and provides a direction to move.
Black and white photo of Sir John Ambrose Fleming
Sir John Ambrose Fleming was the inventor of the thermionic tube.

Thermionic Tube History

The thermionic tube has gone by many names throughout the timeline of its life. It’s also known as and referred to as a vacuum tube, vacuum diode, Fleming valve, and kenotron. Sir John Ambrose Fleming (hence the name “Fleming valve”) invented the thermionic valve and was granted patent GB 24850 in 1904. While there had been computers prior to 1904, the invention of vacuum diodes allowed for the first generation computers to rely more heavily on electricity than they did mechanics. It would be almost half a century before thermionic tubes would be replaced by other technology (see the integrated circuit created by Robert Noyce and Jack Kilby), and many years after that for those technologies to become available to the common individual. 

Fleming's thermionic tube
Fleming’s thermionic tube.

Thermionic Tube: How It Worked

Despite being a technological marvel at the time of its creation, the vacuum tube is a relatively simple design. If one were to look at a diagram of a traditional vacuum tube it would only consist of three components: an electrode, anode, and cathode. The cathode releases free electrons into the Fleming valve, the anode captures those electrons, and the electrode facilitates that transfer of energy. Basically, the device gets the electrons moving and gives them the proper direction in which to move. 

Quick Facts

The patent for the first thermionic tube (which would later be used in first generation computers) was issued in 1904 to one Sir John Ambrose Fleming.
Sir John Ambrose Fleming
Original Use
The thermionic tube utilizes electrical characteristics to allow the flow of free electrons from one end of the device to another. It’s a simple device whose diagram consists of just three components: an anode, cathode, and electrode.
The original cost of the vacuum tube is unknown.
Vacuum tube, or thermionic tube
The vacuum tube is glowing bright red.

Thermionic Tube: Historical Significance

Because of vacuum tubes, vacuum diodes, kenotrons, whatever someone wants to call them, technology would not be the same without them. The Fleming valve revolutionized how electricity could be transported within machines and technologies. The vacuum diode is so important to the history of technological advancements that without it several electronics would not have been possible: radios, televisions, radar technologies, etc. All of these relied on vacuum tubes to act as resistors and transport the correct amount of electricity to each electrical component they use. Even though vacuum tubes have since been replaced with integrated circuits to reduce the size and increase the lifetime of the parts, the kenotron paved the way for the creation of many technologies throughout the timeline of computers and other electronics.

Up Next…

Want to find out more about computer components and some history? Check these reads out below!

  • Integrated Circuit (IC) Explained — Everything You Need to Know. Moving on from the vacuum tube, let’s take a look at the integrated circuit.
  • How to Build a PC: All the Necessary Components. Ready to get your hands on some PC parts and get building? Start with this article.
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Frequently Asked Questions

What is a vacuum tube?

A vacuum tube is a device built to control the flow of charged electrons in a unidirectional flow. In other words, the vacuum tube ensures electricity only travels one way through a device. 

Which was the first vacuum tube?

The Fleming valve is the first vacuum tube, which was invented by Sir John Ambrose Fleming in 1904. 

What is the history of the vacuum tube?

The vacuum tube was invented in 1904 and was the primary method for controlling electric flow through electronic devices for fifty years until the first integrated circuits were invented and used. 

What was the first vacuum tube used for?

Vacuum tubes have always been used to control the flow of electricity through electronic components. 

Who discovered the principle of the vacuum tube?

Sir John Ambrose Fleming discovered the principle of the vacuum tube and was granted a patent for his device diagram in 1904. 

How do vacuum tubes work?

Vacuum tubes work by utilizing three components to control the flow of electricity. A cathode releases free electrons, and an electrode helps “guide” the electrons to their final destination within the device, an anode, which captures those free electrons.

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