If you "Google" valve, you will see information regarding devices for controlling liquids and gases. In the UK the 'electronic valve' is the name given to the vacuum tube or thermionic valve. Valves come in many shapes and sizes and although the envelopes may look similar the contents can vary tremendously.
The development of the thermionic valve or vacuum tube started with the electric light bulb, dating back to the early 19th century. During this period a replacement for candles and oil lamps was a distant dream. Electricity was only available from batteries and static electricity generators. Michael Faraday's (1791-1867) work on electromagnetic induction and, in 1831, the power generator, was to provide a constant supply of high voltage electrical current, making the possibility of the electric light source a reality. Unfortunately there were two basic problems; first, creating an absence of atmosphere (a high vacuum) to stop a heated filament from burning and secondly, when wire was heated in a vacuum by an electric current it melted.
Many attempts were made at a light bulb but none were commercially viable, until around 1860 Joseph Wilson Swan (1828-1914) used a carbonised paper filament however, because of a poor vacuum and a lack of an adequate electricity supply its success was short lived. By 1878 Swan had a patent for a lamp using a Carbon Arc but this had high power consumption and limited life. Finally in 1880 he was successful and obtained a patent for the first practical light bulb using a filament.
In 1879 Thomas Alva Edison (1847 -1931) obtained a patent for a bulb with a carbon filament and a high vacuum, the lamp's life was 13.5 hours! The inside of the glass blackened during Edison's experiments with carbonised filament lamps, this was one problem that persisted, but was to have consequences for the development of valves.
In 1881 the Savoy Theatre, London became the first public building in the world to be lit by electric light.
In the Scientific gallery of the website you can see many early experimental devices for demonstrating the effects of high voltages on different gases, liquids and elements, namely Geissler & Crookes discharge tubes.
In 1884 when Johan Wilhelm Hittorf (1824-1914) was working with hot cathode gas discharge tubes he noticed that the device would conduct an electric current with the applied voltage in one direction but not in the other, he had discovered the "Diode" but did not recognise its significance. In 1880 Edison noticed that the blackening of carbon filament lamps had an area that remained clear, it consisted of a stripe running down the glass directly in line with the two vertical wires that formed the filament, but only on one side.
Electricity in those days was predominately direct current, not alternating as it is today, he suggested that carbon was thrown from the negative side towards the positive but was screened by the bulk of the positive wire, hence the clear portion. Edison positioned a plate between the wires to screen off the carbon, but this did not cure the blackening problem, he did notice however that a current would flow to the plate when it was positive, but not the other way round. This was called the "Edison Effect" also sometimes referred to as Thermionic emission. He filed a Patent for a device for 'Regulating Power Lines' not a Diode in 1883, the first electronic Patent in the US.
Enter John Ambrose Fleming (1849-1945). Fleming was aware of Edison's work and also worked on the blackening bulb problem, working as a consultant to the Ediswan Electric Light Company in London, also to the Marconi Wireless Telegraph Company. Whilst working on the Poldhu transmitter for the first telegraphic communication across the Atlantic, he identified the receiver as being the weakest link.Because he was hard of hearing he wanted a visual indication of the presence of a wireless signal.Detecting (rectifying) the signal was then achieved by the Coherer or a magnetic detector. Using a Coherer reception was either erratic or insensitive. In October 1904 he realised that the "Edison Effect" might help. He recovered one of Edison's bulbs with the additional plate and rigged up an experiment. The bulb used as a rectifier (detector) in the receiver was much more efficient, the Thermionic Diode had been born. Here is his commercial equivalent of his invention.
In 1906 Lee De Forest added a third electrode to his version of the Fleming Diode. His first attempts met limited success; changing the third electrode from a plate to a grid of wire improved the valves control over the electron flow. De Forest called his development the "Audion", he had claimed this device could amplify, probably not true, but the new grid certainly helped. It was not until Dr Arnold of Western Electric USA perfected the device and used it as a Telephone repeater between Washington and New York in 1913 that amplification was achieved.
Using JJ Thompson's theory of electron movement, the workings of a valve are easily explained. The word electron did not appear until it was identified by JJ Thompson in 1897 (1856 -1940). If an ordinary lamp filament is heated and a second plate is placed inside the vacuum with a much higher positive voltage applied to it a current (electrons) will flow from the filament to the plate. When the polarity of the plate is reversed no current will flow. Put a grid of wires between the two elements and to this third electrode apply a voltage more negative than the cathode (filament) whilst the anode (the plate) is more positive. The electrons will be blocked by the grid, turn the grid slowly more positive, and electrons begin to flow from cathode to the plate (anode) increasing as the grid becomes less negative. You have an Electron 'tap', a Valve in which a small change in voltage on the grid makes a large change in current between the anode and cathode.
De Forest's Audion was a poor amplifier but worked well as a detector, we know this device as a "Triode" (three electrodes). By 1901 Marconi's Wireless Telegraph Company was the dominant force in wireless. Captain H.J.Round was Marconi's expert on the valve. Round developed the Triode, including a Gas Control system, the tube at the top is filled with Asbestos which when heated by a match would restore the internal gasses. Valve manufacture soon became profitable, British Thompson Houston (BTH) copied the French TM Valve, developed in 1914 by Telegraphie Militaire, with their R2 (similar to this valve). The "R" stood for Receiving. AEG developed their "A" valves in 1916 which was used as a Telephone repeater amplifier. In Britain the Osram-Robertson Lamp Company were making the BTH version at Hammersmith after WW1. All these valves were hand made, it was not until the new Marconi Osram Valve Company launched their version of the "R" that mass production started around 1917 and by the end of WW1 4000 units were being produced per week, for example the RAF Valves of 1918 and the "R" valve of 1922.
Over the centuries creating a vacuum has posed a problem. The vacuum pump originates as far back as 1670 when Otto Von Guericke in Magdeberg demonstrated that two teams of horses could not pull apart two hemispheres evacuated of air. Without the vacuum, research on electrical behaviour was impeded. Geissler required a pump for his research and developed a machine in 1855. He initially obtained 0.1 Torr and with help and modification 0.01 Torr. Ten years later Dr Herman Sprengel introduced a different form of Mercury pump that achieved almost 0.0005 Torr. This invention was to help Edison with his lamps, but further improvements were necessary for modern valves so the research continued. A low vacuum tube is called a 'Soft' valve, a high vacuum is a 'Hard' Valve. 'Soft' is also used for a faulty or leaky valve. Other gasses like Nitrogen or Helium can be used to effect in valves, 'Soft' valves and gas filled valves are designed with specific advantages.
Even with the very best of pumps some atmosphere will still remain; to remove this 'Gettering' is used. Inserting a material into the valve and heating it helped to absorb any remaining gases inside the valve. This caused discolouration inside the glass, Magnesium silvered the glass, Phosphorous made it red, others like Barium would simply blacken it, Zirconium is also used.
Valves have largely been replaced by transistors, but the latter are not as robust, suffering damage from lightning and static. One transistor, the size of a match head, can replace a normal triode valve.