Electrical pulses, passing through a quartz crystal, are converted into mechanical vibrations. In a mercury delay line store, the vibrations travel through mercury at a far slower speed than would the original electrical impulses. They are reconverted by a second crystal at the far end, amplified and fed back to the beginning again. In this way hundreds of pulses can be stored, ie, kept continuously circulating in a relatively small space, recirculating about 1000 times per second.

Eventually EDSAC had 32 long delay lines, or mercury tanks, and two short ones (example shown here).

Working at Birkbeck College, London, A D Booth invented the principle of data storage on the surface of a rotating metal cylinder of drum, coated with a magnetisable material. Shown here is the first experimental model, in wich can be seen the single brown strip of material round the drum, with the driving motor on the right.

Magnetic drums quickly became the earliest magnetic storage system in widespread use. Booth incorporated them in a serues of computers built at Birkbeck, one of wich became a basis for the HEC cmputers made by British Tabulating Machine Co.