Twistor memory is a form of computer memory formed by wrapping magnetic tape around a current-carrying wire. Operationally, twistor was very similar to core memory. Twistor could also be used to make ROM memories, including a re-programmable form known as piggyback twistor. Both forms were able to be manufactured using automated processes, which was expected to lead to much lower production costs than core-based systems. Introduced by Bell Labs in 1957, the first commercial use was in their 1ESS switch which went into operation in 1965. Twistor was used only briefly in the late 1960s and early 1970s, when semiconductor memory devices replaced almost all earlier memory systems. The basic ideas behind twistor also led to the development of bubble memory, although this had a similarly short commercial lifespan. Core memory Construction In core memory, small ring-shaped magnets - the cores - are threaded by two crossed wires, X and Y, to make a matrix known as a plane. When one X and one Y wire are powered, a magnetic field is generated at a 45-degree angle to the wires. The core magnets sit on the wires at a 45-degree angle, so the single core wrapped around the crossing point of the powered X and Y wires will be affected by the induced field. The materials used for the core magnets were specially chosen to have a very "square" magnetic hysteresis pattern. This meant that fields just below a certain threshold will do nothing, but those just above this threshold will cause the core to be affected by that magnetic field. The square pattern and sharp flipping states ensures that a single core can be addressed within a grid; nearby cores will see a slightly different field, and not be affected. Data retrieval The basic operation in a core memory is writing. This is accomplished by powering a selected X and Y wire both to the current level that will, by itself, create ½ the critical magnetic field. This will cause the field at the crossing point to be greater than the core's saturation point, and the core will pick up the external field. Ones and zeros are represented by the direction of the field, which can be set simply by changing the direction of the current flow in one of the two wires. In core memory, a third wire - the sense/inhibit line - is needed to write or