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In this video about the EDSAC replica, he talks about the mercury delay lines which were used in the original EDSAC machine. They're unreliable, toxic, and relatively expensive.

Delay lines can also be made from wire, which I imagine is much cheaper and maintainable. and the video demonstrates one made from wire. My question is: Why were early delay lines made from tubes of mercury?

  • I've created the tags edsac and delay-lines. – Wilson Dec 6 '17 at 18:39
  • Just so you know, you can use the syntax [tag:tag-name] to make tag-name. (It looks prettier in posts.) – wizzwizz4 Dec 6 '17 at 21:58
  • Oh that's handy, thanks for the tip @wizzwizz4. – Wilson Dec 6 '17 at 22:00
  • Just as a point of interest to those searching the net, CSIRAC also used mercury delay lines. CSIRAC can still be seen at the Melbourne Museum. – Matt Lacey Dec 7 '17 at 5:07
  • Liquid mercury isn't toxic, or at least not toxic to humans who don't ingest it. It's toxic if you heat it so that it starts to evaporate. It's toxic to fish if you pour it down the drain. The mindset in 1947 was that they didn't worry about environmental stuff or low-probability health risks. – Ben Crowell Dec 19 '17 at 21:41
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To quote the wiki:

Mercury was used because the acoustic impedance of mercury is almost exactly the same as that of the piezoelectric quartz crystals; this minimized the energy loss and the echoes when the signal was transmitted from crystal to medium and back again.

Mercury was chosen over other techniques for the first data-delay lines because they were an adaptation of existing delay line technology used for radar.

  • Welcome to Retrocomputing Stack Exchange. Please read the tour. This is a good answer; is the non-quoted section from your own knowledge? – wizzwizz4 Dec 6 '17 at 20:29
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    Another key point here is that these are acoustic delay lines. The question compares them to wire, which would be an electric delay line; getting long delays with a wire would require it to be prohibitively long. – Pete Becker Dec 6 '17 at 20:58
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    @PeteBecker, wire delay lines are also acoustic, not electric: you send a torsion wave down the wire, and read the twist as it reaches the other end. – Mark Dec 6 '17 at 21:12
  • Non-quoted bit is a paraphrase of the linked source. – Alex Hajnal Dec 6 '17 at 21:23
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    @Mark um, that's not an electric delay line though. Electric delay lines are what the name suggests, long (epoxy enacased) wire coils or trace squiggles intended to delay the signal propagation for purposes of timing or alter the transmission line's characteristics. They're less relevant on older tech, where timing isn't nanosecond sensitive, but you can easily slow a signal from >0.8c to 0.6c and lower just by getting the right transmission line material. – Kaithar Dec 7 '17 at 0:02

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