Examples are mu-metal, permalloy, and supermalloy. Copper, permalloy, and bismuth also make nanowires. Mu-metal was developed by adding copper to permalloy to improve ductility. The original twistor system used permalloy tape wrapped around a 3 mil copper wire. The resulting field was greater than the write strength, causing the permalloy state to flip. Electron lenses are manufactured from iron, iron-cobalt or nickel cobalt alloys, such as permalloy. In 1914, Gustav Elmen discovered permalloy, a magnetic nickel-iron annealed alloy. The iPhone 3GS has a magnetometer, a magnetoresistive permalloy sensor, the AN-203 produced by Honeywell. Permalloy is used in transformer laminations and magnetic recording heads. Permalloy was initially developed in the early 20th century for inductive compensation of telegraph cables.