Back in the days when a computer filled a room, or in some cases several rooms, computer memory was very different from modern semiconductor memory. In the 1940’s and 50’s it was a massive engineering challenge to be able to store and recall the information that early computers could operate with. Each solution had their own technical challenges and relative advantages. Here are a few:
- The magnetic drum, the principle for which was actually patented back in 1932, consisted of a rotating drum with a series of magnetic strips each of which had a head to write and read the magnetic state of that strip.
- Mercury delay line memory. Tubes full of mercury take a time to pass vibrations from one end to the other as sound propagates slowly through the dense liquid. Pulses of binary data were input at one end, and read at the other. The output was amplified and sent back to the input so that the compression waves of data were circulated repeatedly through the mercury, to ‘hold’ the information.
- For small amounts of relatively fast to access memory, banks of radio style vacuum tubes were used, this was incredibly expensive to implement and did not scale to large amounts of storage, reliability of the valves was also an issue.
- The Williams-Kilburn tube was a type of cathode ray tube, using the same technology as early television tubes to store the binary data as points of light in the phosphor on the tube.
With the invention of magnetic core memory and early transistors, these earlier forms of memory were displaced. The magnetic drum, being both robust and practical, evolved into magnetic disks, which were then miniaturised to become the disk drives we are familiar with today. Magnetic storage is now increasingly displaced by semiconductor memory storage devices, the currently reigning technology being flash drives such as the little ssd cards that fit in phones and media players. It will be interesting to see what will displace them.