Delay line memory
Memory in which the bits are not stored but travelling: sound pulses sent through a tube of mercury and fed back in when they arrive.
The technique came from radar, where echoes had to be delayed for comparison, and J. Presper Eckert carried it over into computer building. A metre-long column of mercury delays a sound pulse by about a millisecond, enough to keep a thousand-odd bits in flight at once. At the far end the pulse is read, amplified and sent back into the tube — the information exists only while it is moving.
EDSAC and UNIVAC I built their memories this way. The drawback is serial access: a number can only be read when it happens to come round, and temperature affects the speed of sound, so the tube must be held at an even heat. Core memory made the design obsolete in the mid-1950s, though magnetostrictive variants survived in calculators and terminals into the 1970s.