In invention two aspects are closely linked, Need and Technology. The technology of the day determines what can be done, and the need establishes the effort to be expended.
The invention of the triode valve (1905), by Lee de Forest, marks the occasion when electrical technology took a deep breath and suddenly became electronics. The triode provided a means of amplification of small electrical signals. This opened the way for audio amplification and radio, and eventually made practicable many previously unimagined applications.
The triode, shown here in schematic form, takes very small electrical signals connected to Vg, and amplifies them to makes a larger, but similair signal. on Va. The amplified signal can be used to drive, for example, a loudspeaker and make what might previously have been a tiny, inaudible signal from a microphone into something large enough to be useful.
Previously, when mechanical engineering had reigned supreme, inventors had sometimes attempted to implement devices that pushed the technology beyond what was realistically practicable. The story of Charles Babbage and his efforts to develop a programmable, mechanical computer has been told many times.
In the days before electronic amplification recorded sound using mechanical means had limited success. Television, in its first versions, used mechanical scanning which was soon superseded by electronic systems. These technologies were driven by a need which could not quite be met by the technology of the day. The principles were sound, but in practice the limitations of mechanical systems were soon evident.
The triode was a crucial turning point, it was a practical means of amplification of high frequency signals. The crucial aspect is high frequency. Mechanical systems can provide a measure of amplification. Levers and pulleys are ways of rescaling or amplifying physical movement, but inertia limits how quickly a physical system can move. All mechanical systems are doomed to relatively low speed operation.
The electronic amplifier using triodes and similair components was soon joined by a variant, the electronic oscillator. (Oscillators are a key element of radio technology.)
When the first radars started to appear, they were built using the established elements of radio: amplifiers and oscillators. A third element was added, electronic switching. The first computers, when they came along, needed the new electronic switches in huge numbers.
The numbers required started to reach the limits of the viability of valves and a replacement was needed. The transistor arrived at just the right time. Transistors could be used as amplifiers, oscillators or switches. They were smaller, used less power, and were soon much more reliable. Moreover, transistors lent themselves to implementation in bulk as integrated circuits, which led to further improvements in size, cost and reliability.
As the new technology matured, other parts of the computer were re-engineered. The first computers had been a grab bag of borrowings from other devices. The first memory systems had used delay lines, which had previously been used in ground based radars. (Many of the early computer designers had worked on radars and knew how to make such esoteric technologies work.) As soon as it was possible delay lines and other weird and wonderful memory systems were replaced by semiconductor memory. At present the spinning magnetic disc is bravely hanging on to its traditional role as a non-volatile memory back up but it can’t hold out much longer.
Computer technology is now so highly developed that a computer is not always an end in itself. It’s often a component within a bigger system. Radios and radars, which had previously contributed technology to computers now contain, as components many computers.
The development of computers has taken place while electronics emerged from the stone age and is still driving the cutting edge of technology development. For a while though, quite a number of complex computers were developed using mechanical systems and stone age electronics. These were analogue computers but that's something for another blog.