continues until the twistor strip is "used up", forming a compact cube of memory. Along one side of the memory, connected to each of the solenoid loops, was a series of small cores used solely for switching (their original purpose, development as a memory came later). The main reason for Bell's development of twistor is that the process could be highly automated. Although the folding process that completed the twistor might be carried out by hand, the layup and laminating of the sheets was easily handled by machine. Improved versions of twistor also wrapped the section of bare copper initially used solely for the return path, thereby doubling density without any changes to the production techniques. Operation Writing to twistor was effectively identical to core; a particular bit was selected by powering one of the twistor wires and one of the solenoid loops to one half of the required power, such that the required field strength was created only at the intersection of the two. Reading used a different process. Unlike core, twistor did not have a sense/inhibit line. Instead, it used a larger current in the solenoid, large enough to flip all of the bits in that loop, and then used the twistor wires as the read line. Twistor was thus read and written one plane at a time, rather than in core, where only one bit per plane could be used at once. Permanent magnet twistor Twistor could be modified to produce a ROM that could be easily re-programmed. To do this, one-half of each solenoid loop was replaced with an aluminum card into which tiny vicalloy bar magnets were embedded. As the solenoids have to be complete circuits in order for current to flow through them, they were still inserted as folded sheets, but in this case the loop was inserted between the folds of twistor instead of around them. This allowed the single sheet to act as one half of a solenoid loop for two folds of the twistor, above and below. To complete the loop, the card of magnets was placed on the other side of the twistor tape. Reads were performed by powering the solenoid to a point about half of that needed to produce a write. This field was "reflected" by the aluminum sheet, closing the loop, magnetically. The resulting field was greater than the write strength, causing the permalloy state to flip.