Drum memory
A rotating metal cylinder with one read head per track — the main memory of the 1950s, where fetching a number took as long as the drum felt like.
Gustav Tauschek patented the principle in 1932: a cylinder with a magnetisable surface spinning under a row of fixed heads. Since the heads never move there is no seek time, only the wait for the right patch of surface to come round. At twelve thousand revolutions per minute the average was a few milliseconds — slow, but faster and cheaper than the alternatives before core memory.
It gave rise to a lost programming craft. On machines like the IBM 650 you placed instructions at intervals around the drum so that the next one passed under the head just as it was needed; the SOAP assembler did it automatically. Bad placement could make the same program ten times slower.