Quicksilver memory

by Jo Brodie and Paul Curzon, Queen Mary University of London and Adrian Johnstone, Royal Holloway, University of London

Silvery liquid twisting as it flows downwards.
Image by Roman Shashko from Pixabay

Some 1950s computers used tubes filled with mercury as a memory to store numbers. Mercury is a metal that is liquid at room temperature. It’s also known as quicksilver as it flows very easily, but in computing it was actually used to trap information.

Early computers needed a way to store data that would survive indefinitely, even if the computer was stopped. ‘Delay lines’ provided the solution. Data arriving electronically at a mercury delay line struck a converter (called a ‘transducer’) which converted the information to a sound pulse in the mercury. The sound travelled through the tube at the speed of, yes, sound and when the pulse reached the other side it hit another transducer and was returned to its electronic form. That might not sound (sorry) like much of a delay but compared to the speed that an electrical signal moves through a wire (a fraction of the speed of light), it’s like a gentle stroll. Once inside the mercury tube the sound pulses could be looped back and forth, safely ‘parked’ until needed. The computer would use its clock to help it count how many pulses had passed and a microphone listened for the right time to release it from the memory store back into the circuitry to do a calculation with it.

Mercury is expensive, so computer pioneer Alan Turing recommended using gin instead. He claimed its mix of alcohol and water gave perfect properties for the job while being so much cheaper.

Think about tennis serving machines that shoot balls at you. If you put one in a squash court, then a ball being fired will bounce back and forth off the walls but quickly drop to the floor. A delay line works like having two machines facing each other. One fires a ball so that it hits a lever (the transducer) which tells the other machine to fire a ball back, which then hits a lever on the first machine… and so on. Now there is always a ball in flight (a pulse in the delay line) because the motion of the original ball is detected, and used to make a new ball (pulse) that is injected back into the system. Start the first machine by making it fire balls in an initial ball-no-ball pattern and the system stores that pattern, that information. Using cunning contraptions, motion can keep information firmly in one place.

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This article was funded by UKRI, through Professor Ursula Martin’s grant EP/K040251/2 and grant EP/W033615/1.

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