Magnetic core memory was the last--and probably most mainstream--data storage technique to rely on magnetization and positive/negative charges to read and write binary data in the decades before the disk drive fully emerged. Small magnetic rings were threaded with wires and formed grids that read and wrote data. Each ring was a core that represented a single bit of data--either a 0 or a 1. Engineers strove to decrease data access time and increase data access rates by using many grids of cores at once. Core memory could store data without power and was at least somewhat impervious to radiation (important features for use in military systems).