Bletchley museum treasures vintage tech

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Ferrite core memory

Ferrite core memory
Invented in 1955, ferrite core memory was the mainstay random access storage medium for mainframe and minicomputers throughout the sixties. It works by hundreds or thousands of tiny O-shaped ferrite rings — ferrite being a iron-containing ceramic — that can be magnetised when current passes through two wires threaded through their centre.

This magnetism can be read back by another wire — the sense wire, which also passes through the core. However, to get this signal, the ferrite has to be demagnetised by a pulse of electricity through the first wires. Thus, once read, the ferrite has to be reset back to its original state. Density is low, with one bit per ferrite, but planes of core memory.

When not being read, however, ferrites were non-volatile; you could switch a computer off overnight and turn it back on with memory contents intact. The first solid-state memory replacements for core store had huge batteries to try and replicate this behaviour, until it was realised that reloading off tape was almost always more sensible.

Photo credit: Rupert Goodwins