01 · The weave
Every crossing of two wires threads a ring
A core plane is textile before it is electronics. Copper X wires run the rows, copper Y wires run the columns, and at every crossing both are threaded through a bead of ferrite — a hard magnetic ceramic, a couple of millimetres across in the early planes. Magnetised one way round its ring, the core is a 1; the other way, a 0. Nothing moves, nothing glows, and nothing needs power to remember: cut the mains and the word on this plane would still be here next year.
This demonstration plane weaves 32 × 16 = 512 cores and patrols the seven rows that carry the word. Production planes ran 64 × 64 and were stacked into blocks — one plane per bit of the machine word — hand-threaded, ring by ring, mostly by women working under magnifiers.
02 · Coincident currents
Half a current is a nudge; two halves are a flip
Ferrite’s trick is a nearly square hysteresis loop. Drive half the switching current down one X wire and half down one Y wire, and only the single ring at their crossing feels the full field. Every other ring on either wire gets a half-select — an excursion along its loop that falls short of the coercive knee and relaxes back to where it was. That is how one ring out of thousands is addressed with no switch at the ring itself: the selection logic is the material.
The B–H loop beside the plane is not an illustration. It is the working point of the addressed core, traced live: watch it ride out to the ½ tick and come home on a half-select, and snap across the knee when two currents coincide.
03 · Reading is destruction
To read a bit, the machine tries to erase it
There is no way to ask a core what it holds. You can only try to write a 0 into it and listen. If it held a 1, it flips — and the collapsing flux induces a pulse, tens of millivolts in period hardware, on the sense wire that threads every core in the plane. If it held a 0, near silence: in this model the read-zero blip is about 7% of a read-one pulse. Either way the core now holds 0. The act of reading erased the bit.
So every read ends in a rewrite: the same pair of wires drives the opposite way and puts the 1 back before anyone notices it was gone. That destructive-read-then-restore heartbeat — running ring by ring across the plane above — took a few microseconds, by the mid-fifties, and it is what “memory cycle time” originally meant.
04 · The disturb problem
A thousand harmless nudges are not harmless
A half-select is supposed to leave a core exactly where it was. But no real loop is perfectly square: each worst-case, one-sided half-select walks a little remanence out of an unselected core. Flip the disturb stress switch on the panel and the plane hammers one ring’s row with same-polarity half-selects, on the pattern real engineers used for worst-case testing. Watch its working point creep down the B axis until — never once selected — it lets go of its bit, and the word on the plane is quietly wrong.
The demonstration compresses the physics: here a ring gives way after about two dozen worst-case pulses, where a production core was specified to shrug off vastly more. In normal operation reads and rewrites drive opposite polarities and largely cancel — which is exactly why disturb testing used deliberately one-sided patterns. The mechanism is the one that kept memory engineers awake, and it is why every core batch was margin-tested before a single plane was strung.
05 · Provenance
Twenty years as the memory of everything
An Wang showed in 1949 that a magnetic core could be switched and sensed by pulses; Jay Forrester’s coincident-current array put cores at every crossing of a wire grid, and by 1953 it was running as the main memory of MIT’s Whirlwind. From roughly the mid-fifties to the early seventies nearly every computer in the world kept its working memory in woven ferrite — until semiconductor RAM undercut it on price, and “core” survived only as the word programmers still use for a memory dump.
The rendering above is a faithful simplification: rings sit at alternating 45° so both drive wires clear the aperture, the sense wire threads every core (serpentine here; often diagonal in period planes), and the hysteresis model is a scaled tanh pair — square enough to select with, imperfect enough to disturb.