Delay-line memory
Early computer memory that kept bits alive as pulses travelling through a tube, caught at the far end and sent round again.
Delay-line memory stores information by keeping it moving. In the mercury version used by EDSAC and LEO, a quartz crystal at one end of a tube of mercury turns each electrical pulse into a tiny click of sound. The click travels down the tube and a second crystal turns it back into electricity, which is cleaned up, amplified and sent straight back to the start.
Sound moves through mercury at about 1,450 metres a second, so a trip down a tube about 1.5 metres long takes roughly a thousandth of a second. That is slow enough for hundreds of pulses, ones and gaps, to be in flight at once, like carriages on a train. An everyday comparison is a juggler: the balls are only safe while they keep going round.
Because the bits circulate, a program has to wait until the number it needs comes round to the read-out end. The speed of sound in mercury also changes with temperature, so the tubes were kept in boxes at about 40 °C.
A common misconception is that old memories were slow because the electronics were slow. In fact the waiting for the right pulse to arrive was often the bigger delay. Today's DRAM chips forget too, and have to be refreshed thousands of times a second.
