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one through a dense medium – such as a tank of mercury – so that they travelled relatively slowly. When each vibration reached the other end, a piezoelectric crystal picked it up, converted it back into an electrical impulse and sent it back to the start again. Delay-line memory was a refreshable memory, and as opposed to modern RAM, it was serial access.
To access a certain a few milliseconds until the relevant vibration reached the end of the tank. Delayline memory also required complex equipment to focus the vibrations so that they didn't reflect off the inside walls of the tank and cause interference. Because of this, delay-line memory was too bulky and limited to survive.
Both the Selectron and the Williams Tube were superseded in the market by a far more convenient and cost-effective form of main memory that was about to take the world of computing by storm.
Hard core
From its introduction in the early 1950s, so-called 'core' memory quickly became the dominant form of technology. Non-volatile and cheap to make, it survived well into the late '70s – even beyond the introduction of DRAM chips.