Thermionic valves are not a new technology and are now largely redundant thanks to solid state technology [transistors]. However, during their early development evacuated tubes [precursors to valves] provided evidence which led to the discovery of the electron and revised theories of the nature of light. Much of present day technology such as telephones, broadcast radio and computers have origins tied to thermionic valve. In present day society, thermionic valves are rarely used, with exceptions of specialised laboratory experiments, some Hi-Fi components and most high powered instrument amplification.
The thermionic valve [also called a vacuum tube] was a culmination of many years of experimentation using evacuated glass tubes and 'cathode rays'. 'Cathode rays' are streams of charged particles that can be seen between high voltage electrodes in an evacuated tube. 
The specific nature of cathode rays was a source of much scientific enquiry in the late nineteenth century and many special-purpose evacuated tubes were manufactured to attempt to determine their properties. One major property of cathode rays, which made many other experiments easier, was their ability to excite and cause a phosphor to glow. Accordingly many experiments utilised plates or screens coated in zinc sulphide to observe the paths taken by the cathode rays. 
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Maltese cross tube-
The Maltese cross tube proved both that the cathode rays travelled in straight lines and that they originated at the cathode [negative electrode]. The manner in which the cathode rays were emitted and travelled in straight lines from the electrode were similar in nature to light, as they cast a shadow from the Maltese cross, it appeared that they were stopped by solid objects, another property of light. 
Paddle wheel tube-
The paddle wheel tube included a wheel with vanes that was able to turn on a central pivot. When cathode rays were directed at one side of the wheel, it turned. This proved that the cathode rays were a stream of charged particles that could impart momentum in a collision. 
A tube where a beam of cathode rays was directed along a screen-
Directing the beam of cathode rays along a screen of phosphor coated material allowed their path to be easily observed. This made it easy to observe magnetic deflection. This property also proved that cathode rays were charged particles, as they were deflected due to the motor effect. Early attempts to electrically deflect cathode rays however were unsuccessful, yielding no result. This led to much controversy as to whether they were indeed charged particles or if they were some kind of light.
Later experimentation by JJ Thomson produced electric deflection [using a better vacuum] and found the charge to mass ratio of these charged particles. Thomson concluded from his null method experimentation that these particles had either the same mass as hydrogen ions [protons] and 1800 times the charge or the same charge and one 1800th the mass. The second option he inferred was correct, and the new particle was named the electron. 
The Thermionic Valve
Before cathode rays were fully understood some interesting phenomena had been produced using cathode ray tubes. Electric and magnetic deflection had been produced, along with rectification, observed by John Ambrose Fleming and developed into the thermionic diode [oscillation valve] in 1904 . He observed that an alternating current was passed through the valve allowed only one polarity of current to pass through, like a valve.
The principle behind the thermionic diode is relatively simple. [fig 1][fig 2] as the cathode of the diode is indirectly heated the potential difference for thermionic emission is reduced. Thus if a negative voltage is applied to the heated electrode, a current may flow, but if the same current is applied to the diode in reverse, the voltage that is required for current to flow is much greater, as it needs to emit electrons unaided. Due to this, at appropriate operating voltages, diodes may be used to rectify current. 
Fleming saw a use for the diode for a means of rectifying the 'feeble' alternating currents, and producing a DC current, which was much easier to use.
Following the patenting of both the thermionic and solid state diodes by 1905  the introduction of a third electrode into the vacuum tube was achieved by Lee De Forrest and patented as the 'audion' in 1907. . Forrest's 'audion' [fig 4] was a similar design to many thermionic diodes; however the addition of a third electrode, taking the form of a fine grid between the anode and cathode, was a crucial development of useful thermionic valves. .
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Though developed and patented in 1907, the audion's full potential was not realised until 1911 , indeed, De Forrest intended the audion as a leaky grid detector . The major breakthrough of the audion was its ability to be used in amplification circuits. Amplification circuits use a small input signal to create a much larger output signal. The simplest thermionic valve amplification circuit [fig 4] uses a single valve. . This circuit, being based around the operation of a single valve, a good example of the basic context of a valve in operation.
Thermionic valves underwent a great amount of improvement and development in the years between their inception as useful circuit elements to the beginning of the transistor's dominance over them for most applications. These included micro tubes, multiple devices inside one enclosure and other noteworthy advances. .
For the simplicity the concepts in the remainder of this paper will be examined using triodes, as the concepts covered are applicable to valves with larger numbers of electrodes, however are most easily explained in a three electrode valve.
Basic Technical Operation of Thermionic Valves
As the thermionic valve and technology surrounding it improved a great deal was learned about their operation, in order to improve the operation of the devices which relied upon their properties.
As noted in basic history, thermionic valves rely upon thermionic emission of electrons for a negative electrode inside an evacuated tube. An anode within the same tube with a positive charge will allow current to flow. This principle is employed in thermionic diodes [fig 2] . As already described, a thermionic valve is a thermionic diode with the addition of a third electrode between the anode and cathode. This extra electrode takes the form of a fine grid that surrounds the cathode. [fig 6]. A voltage applied to this grid, depending on polarity and strength, will allow more or less current through the main carrier of the valve [anode to cathode] this small signal on the grid can be from a number of sources, such as microphones or instruments in the case of amplification, or other elements in a much larger circuit.
Whether directly heated by high voltage and current, or indirectly heated by a heater circuit, electrons are emitted from the cathode, partially according to the Richardson Dushmann equation, and partly due to an excess of electrons [negative charge].  these electrons form a 'space charge' around the cathode.  If the anode has a positive charge, then it will attract the negatively charged electrons, and hence, a flow of charge [current] will be observed. With the exception of positive-ion current, a movement of positive ions within a specialty or faulty valve, which do not behave in the same manner as 'hard' [ideal vacuum] valves.
This flow of current is typical of both thermionic diodes and triodes. 
The 'grid' electrode, taking the form of a fine coil of wire situated between the anode and cathode, takes the form of a fine coil of wire, closer to the cathode than the anode.  [fig 6]. This additional electrode is the key to the revolutionary nature of the valve, allowing signals to be amplified, and for separate elements of a circuit to interact, switching etc. These functions led to the birth of electronics.
The grid electrode, when a negative charge is applied, serves to limit or stop the flow of electrons between the cathode and anode. The negative charge of the grid repels the negatively charged electrons in the pace charge . This grid voltage can be varied to 'switch' the current, such as used in computing [fig 7]. This process, however, is more pertinent in the use of a valve in an amplifier. The varying signal from an audio, power, or other source, when applied to the grid, correspondingly affects the output current in the same manner as the signal, which can be used to amplify the original signal.
However, as plate or cathode emission is often non linear, circuitry must be designed so as to use the most linear section of the valve's characteristic. An example of a valve characteristic is given in [fig 8]. The use of a linear section of valve operation is to prevent distortion, caused by nonlinear transformations by the valve. This is of special significance to high fidelity audio applications, among other industries. 
The Modern Dominance of Transistors- The conclusion to the Valve Age
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Thermionic valves were used in all electronic devices until the development of cheap, reliable semiconductors occurred in the 1950's and 1960's.  Thermionic valves, in a circuit or device are power hungry, either for heater circuits, high voltage emission, or both, and have a limit to miniaturisation, with the smallest thermionic valves for use in appliances, the general electric 'compactron' [fig 5], which combined multiple devices into one valve, was about 3 cm wide and 6 cm long.  While this does not sound overly big, the transistors used to replace this component would use approximately half the space. Another miniature thermionic device of note is the 'nuvister' these tiny, metal clad thermionic valves were introduced in 1959.  while much smaller than the compactron, it performed only one task, as a triode or tetrode. . When RCA [the 'nuvister's manufacturer] introduced the product, they themselves acknowledged that the transistor was 'a natural for low level applications'. 
Thermionic valves suffer from numerous inherent shortcomings from essential elements of their construction. These integral components limit the miniaturisation of thermionic valves, and hence, their usefulness. Design characteristics that limit minimum size of thermionic valves include; the glass envelope, heater coils or sufficient sized electrodes, as well as the multi-pin mounts or connection wires that commercial tubes are connected with. 
The glass envelope must be of a certain minimum size to form a compromise between size, weight and fragility. A smaller amount of glass limits the first two factors, but may prove too fragile to be useful, particularly at high temperature. The electrodes must either have heater coils, or be of sufficient size to reliably emit electrons without burning out. The fragility and consumable nature of these devices lead to the third factor limiting miniaturisation, the connections. As thermionic valves must be replaced fairly regularly, a clear and reliable method of location and connection must be developed. The typical 'pin' connections found on most valves solve this problem; however this pin array and metal frame, to handle heat expansion, proved a great limiting factor in their development and miniaturisation.
These limits to miniaturisation proved a major drawback to the thermionic valve. As demand for portable electronic devices, as well as smaller and less costly to run devices for the home grew, the clear advantages of the transistor became apparent. They did not need high voltage circuits, were smaller and less fragile, all properties that led to their popularity in portable, and soon after, home devices.
The ultimate failing of the thermionic valve, in a social context, and partially in hindsight, is its almost complete inability to be used in portable devices and computers. While computers such as ENIAC [fig 7] were built, that ran exclusively through valve technology, they were unsuitable, for many of the reasons that the transistor became dominant in almost every other field, with the addition of the difficulty of cooling a room full of thermionic devices, each of which had an operating temperature in excess of 190 degrees Celsius , as well as the task of replacing broken tubes, which may have been due to failings due to overload, physical shock, thermal shock, age or a multitude of other issues. In hindsight, in our computer age, entirely reliant upon the transistor, the thermionic valve while revolutionary was a technology with an inbuilt death clause. They allowed appliances to become advanced and complex enough, that their continued use was not feasible, and they led to the development of devices, such as portable radio transceivers, for which they were barely suited. The thermionic valve itself was its own undoing in an advancing world, and the transistor only provided us with an alternative before one was desperately needed.
fig 1- diagram of thermionic diode 
fig 2- thermionic diode ,  Exhibist Thermionic Emmission
fig 3- Audion 
fig 4- simple valve amplification circuit 
Fig 5- article announcing general electric's campactron 
Particular note on picture and caption
Fig 6- thermionic valve
Fig 7- Valve Computer 
Fig 8- valve characteristic (12ax7)
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