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            In the early 1880's Thomas Edison was experimenting with his invention the electric lamp, he found that if he made his filament negative a current flowed between the heated filament and a nearby positive electrode, called a plate. In 1904 John Ambrose Fleming discovered that by modifying Edison's tube he could rectify an AC current. So he placed a metal cylinder around the filament in the bulb and connected the cylindrical plate to a terminal, the current could only flow from the negative filament to the cold positive plate. This became known as the diode. This not only rectified AC to produce DC but it could also be used to detect radio waves in recently invented radios. Two years later American Lee De Forest modified Fleming's diode so that it could amplify, he called it the triode because it had a third electrode, the grid, between the filament and the plate. The electrons had to pass through this grid so that a small change in the voltage flowing on the grid could produce much larger current flowing through the tube. This meant that a weak radio signal picked up by an aerial could be amplified to a loudspeaker. The invention of the triode, pentode made possible voice broadcasting across America and Europe. .
             These valves were fragile as they were mainly made out of glass, they were used in radios and some of the early computers, but they were not reliable. While normal operation the filament would heat up and cool down quickly as equipment was turned off and on, and this meant that the would be some sort of breakdown in the valve. These valves were also large at time and they used large amount of energy and they would generate a lot of heat. Also because the filament needed to be heated up before it would start emitting the electrons, there was a limitation of its use because of this time delay, when the earliest of computers were fitted with valves they broke down every 15 or so minutes and the valves were time consuming and costly.

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