A small ring, or core, of ferrite (a ferromagnetic ceramic) can be
magnetized in either of two opposite directions (clockwise or counterclockwise). Therefore such a core can be used for storing a bit of information.
Around 1950, memory was implemented by mercury and nickel-wire
delay lines, magnetic drums and 'Williams
tubes'. The invention of core memory in 1949
was a leap forward in cost-effectiveness and reliability; it is generally ascribed to A. Wang and J.W. Forrester .
Core memory was used first in the Whirlwind computer  besides Williams tubes.
The first commercial computer with a core memory (100.000 bits,
cycle time 17 μs) was the IBM 705, a vacuum tube based
computer delivered in 1955. In 1976, 95% of all computer main
memories consisted of ferrite cores, 20-30 billion of which were
yearly produced worldwide . The price per bit of core memory was
20 dollarcent in 1960 and decreased from there with 19% per year.
In 1974 was the 'turn-over' to semiconductor (transistor) memory
with the advent of the 4 kbit chip; the cost was 1 cent per bit for
both techniques by then.
For almost 15 years, 'core' has been the most important memory device.
Core memory has been in use until the turn of the century for special purposes,
because it retains the information when the power is switched off,
and it is resistant against radiation.
The physical basis of core memory is the fact that a current
sent along a wire passing through a ferrite core sets a persisting
core magnetization, if the current exceeds a certain threshold. A
current in the opposite direction will reverse the direction of the
magnetization. In this event a voltage pulse will be induced in
another wire threaded through the core, the 'sense wire'; the
polarity of this pulse is determined by the original magnetization
direction. Clearly reading is a destructive operation, and as part
of the read cycle the original state of each core must be
Core memories were often organized as a planar matrix, the 'write'
wire being split up into two wires (row, column) each carrying half
of the threshold switching current. This made it possible to
address a specific core in the matrix for reading or writing. Accordingly, this type of core memory is often referred to as 'coincident current memory'. A stack of such planar matrixes would make a 3D structure, where sophisticated access schemes were possible by threading the cores in each separate plane by up to 5 wires.
Early core memory was hand-woven, a cumbersome and therefore expensive process. In  is the described how core-threading was mechanized, reducing the time to thread a 64*64 plane form 25 hours to 12 minutes.
1. Middelhoek, George and Dekker: Physics of Computer Memory
Devices. Academic Press 1976.
2. Ivall (ed): Electronic Computers. Iliffe London 1956.
3. MF11-U/UP core memory system maintenance manual. Digital
Equipment Corporation 1973.
4. Redmond and Smith: Whirlwind, the History of a Pioneer
Computer. Digital Equipment Corporation 1980.
5. Wikipedia: Magnetic core memory
6. Bashe et al: IBM's Early Computers. MIT Press 1986. rev March 30, 2017