If the bit was beside an unmagnetized bar magnet in the card, the field was not opposed and the flip caused a current pulse in the twistor wire, reading a "1". However, by magnetizing the bar at that bit, the bar magnet opposed the field being created by the solenoid current, causing it to be below the write strength, and preventing the flip. This read a "0". The permanent magnet twistor (PMT) was re-programmed by removing the plates and placing them over a custom writer. Vicalloy was used because it required much more power to re-magnetize than the permalloy tape, so that the system would never come close to re-setting the permanent magnets while in use in the memory system. The writer system used much larger currents that overcame this resistance. The PMT that was used in the 1ESS system used modules with 128 cards with 2818 magnets (for 64 44-bit words) on each. This produced a module with 8192 words (8 kibiwords). The complete store used 16 modules for a total of 131,072 words (128 kibiwords), equivalent to 720,896 8-bit bytes (704 KiB). Piggyback twistor Another form of twistor ROM replaced the permanent magnet cards with a second magnetic tape wrapped around the first on the twistor lines, in a "piggyback" configuration. This tape was coated with coballoy instead of permalloy, which is much "harder" magnetically, requiring about twice the field in order to flip. To make the system even harder, the coballoy tape was about two and a half times thicker than the permalloy one, so the resulting field strength was five times. The external current required to flip the state of the coballoy tape was about 15 times that of the normal operational current. Read operations in the piggyback are identical to the permanent magnet version. Writes were slightly more complex, due to the fact that piggyback twistors all featured the magnetic tape along the entire length of the X wire. This meant that any one solenoid was wrapping both the bit that is being written as well as the one on the section of return wire. To set the one both and not the other, the solenoid was first powered in one direction and then the other, while the current in the twistor line remained constant. This created two magnetic fields in turn, one aligned with the first section of wire and then the second. All