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Delay Lines

Technician with mercury delay lines

A technician checks out mercury delay line memory for the UNIVAC I computer.

Tech Talk: Delay Lines

Advances in radar during World War II had an unanticipated spinoff: delay lines as computer memory.

Delay lines were developed to store radar blips so that screens displayed only new, moving blips. In computers, delay lines converted data bits (ones and zeros) into sound waves, transmitted them acoustically, then converted them back into bits. They circulated forever until changed by the computer.

Mercury-filled tubes had transducers at the ends to generate and receive bits. In magnetostrictive delay lines, an electromagnet twisted a long wire one way or the other to represent ones or zeros.

Wilkes with EDSAC

Maurice Wilkes designed the EDSAC, one of the earliest stored-program computers, which used delay line memory.

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Ferranti Sirius magnetostrictive delay line

Like all delay line memories, a stream of bits continuously circulated and was accessible only at one point. The medium here was a thin strip of special metal rolled into a coil, with transducers at either end.

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Radar technician at radar display

Mercury delay line memories were initially developed for WWII radar systems.

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Ferranti Sirius

Sirius was a small, low-cost business computer using a simple programming language. Its main memory was a magnetostrictive delay line.

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Deuce delay line memory

Turing’s design made the Deuce fast, but difficult to program. Main memory was 1600 bytes circulating in this mercury delay line.

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UNIVAC I delay line memory central support

A set of these structures with associated electronics gave the UNIVAC I a memory of about 1.5 KB. Each tube weighed nearly 800 lbs when filled with mercury.

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