Introduction
The invention of the vacuum tube or thermionic valve is the dawn of the age of electronics.
Its invention enabled a leap forward for the wireless technology. Many new and exciting applications were found for these new devices. Vacuum tubes were used first as telephone repeater amplifiers and then in many others applications, not always linked to communications. Electronics was born. Actually the history of the vacuum tube might be seen as the first chapters of the history of electronics.
Today, vacuum tubes (or thermionic valves) are rarely seen, only appearing in vintage radio equipment. Even the few areas where vacuum tubes were previously found are now overtaken by semiconductor technology. The only field where tubes are still unmatched is the high frequency high power amplification. That is above 50 MHz, when power involved is in excess of 10 KW. Most obvious applications of this kind are Broadcast Amplifiers (see here an example), Radars, and Particles Accelerators Drivers (for example in a Rhodotron).
How it started
The history of the vacuum tube spans over a century now.
The first vacuum tube was not made until the beginning of the 20th Century. Nonetheless there were hints of its future discovery many years before. Professor Guthrie made one of the first discoveries leading to the thermionic valve in 1873. He was investigating effects associated with charged objects. He noticed that a red-hot iron sphere negatively charged would discharge itself. He also found out that this did not happen if the sphere was positively charged.
In 1883, the American inventor Thomas Edison made the next important observation. Edison was working on electric light systems. One of the major problems he was facing was their short life. The filament life was a problem, but the main limiting factor was that the bulbs quickly became blackened. At first it was thought that it was caused by atoms of carbon from the filament hitting the glass. As it was known that the particles leaving the filament were negatively charged, he tried to prevent those particles hitting the glass. The most successful method that Edison tried was to place a metallic element into the envelope. He reasoned that if this second electrode was made positive, particles would be attracted away from hitting the glass of the bulb. Experimenting with the polarity of the charge on the second electrode, Edison saw that when the second element was made positive with respect to the filament then a current flowed in the circuit. On the contrary, this did not happen when the potentials were reversed. Edison patented what he found, but he did not understand the underlying physics, nor did he have an inkling of the potential value of the discovery. Even so it became known as the Edison Effect. The seed had been sown for later discoveries.