[Pitarou]

I don't think it was standing waves. If memory serves (i.e. I may well have got this wrong) they stored data as a pattern of pulses travelling down a mercury tube.

They borrowed this trick from radar engineers. The engineers needed a way to compare what the radar is seeing now to what the radar saw on its last rotation, so that they could detect moving objects. They did this by sending the information down a mercury delay tube of suitable length.

[jacquesm]

You're absolutely right, standing waves was the wrong term, it should have been travelling pulses. Apologies!

http://en.wikipedia.org/wiki/Delay_line_memory

[icegreentea]

You're right, it's not standing waves, being used a delay line. You make it 'persist' by sending a signal down the delay line, reading it out the other end, and regenerating the signal into the delay line. This is different from a standing wave, since a standing wave results from the reflection of a wave from the ends of its transmission mediums, while in delay line memory, reflections would probably be a really bad thing.

Relatedly, if you ever had an old school analog sound mixer with sound effects, it would have a spring delay line for delay and echo effects. If you were clumsy like me and ever dropped one, you could here it boinging around.