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Mercury delay line
An unusual and marginally practicable data storage system, mercury delay lines were invented for use in second world war radar systems and subsequently saw service in early computers. This one was used in the Lyon's Electronic Office computer, Leo 1, in the early 1950s.
Mercury delay lines store information as a series of ultrasonic pulses sent from one end of a column of mercury to the other. They could store around 500 bits of information, but were difficult to drive and had to be kept in uncomfortably warm surroundings to be efficient.
The same principle, using quartz delay lines, could be found in European colour TV sets until the early 1990s: it's also been proposed that the reflectors left by Apollo astronauts on the Moon could allow the space between it and the Earth to be used as a laser-delay storage system.
Credit: Marcin Wichary/Flickr
An icon of early computing, the punched card was very low density but robust — and could be altered and read by unaided, skilled humans at a pinch.
Ultimately deriving from automated weaving machines of the mid-18th century, IBM was the most high profile user of the technology. The form it used was developed by Herman Hollerith — hence the alternate names IBM or Hollerith cards — for the 1890 US Census; his Tabulating Machine Company subsequently became IBM.
The most common size of card could store around 160 characters, although many permutations were used.
Alongside the punched card, paper tape remains fixed in the public mind as symbolic of mid-20th century computing. Developed as storage for teletypes — huge electromechanical devices that were a cross between a typewriter and a telegraph — paper tape came in variable length and could thus store variable amounts of data.
Like punched cards, the holes in the tape triggered optical sensors which turned the patterns in the paper or plastic back into electrical symbols, 5 bits at a time.
One of the most famous uses of paper tape was in Colossus, the reprogrammable electronic computer used by Bletchley Park to crack high-level German codes in the second world war. By replacing the usual mechanical sprocket synchronisation with an optical method, the machine could ingest data from paper tape at a highly respectable 5,000 characters per second. Although the electronics was capable of more, at higher speeds the paper disintegrated.
Paper tape's last hurrah was among radio amateurs, who used it to control radioteletypes until the all-conquering microprocessor saw them off in the mid-1980s.
Photo credit: Rupert Goodwins