The wrapping around the inner core (spring steel) of most guitar strings are made from a non-ferrous material such as bronze. Bronze is an alloy of copper, tin and zinc. Bronze is not attracted to the magnetic field in a guitar pickup. You can get sounds out of a acoustic guitar string because the inner core of the wound strings that are made of spring steel. The core is the same material used on the B & E strings, and are attracted to the magnetic field in a pickup. Steel is an alloy of iron and carbon and contains between .02 to 1.5 percent carbon.