Gibson used covers on their original humbuckers designed by Seth E. Lover. It was use in combination with the two coils to reduce unwanted 60 cycle hum often associated with single coil pickups. Seth used nickel silver covers to help eliminate unwanted outside electrical interference from entering the coil. The combination of two coils hooked up 180 degrees out of phase and two magnetic fields (north & south) the coils are covered to give maximum coil protection for unwanted electrical interference. The nickel silver cover has less capacitance which can cut the higher frequencies in the pickup. It gives added shielding to the coils and to give the coils a protective and cosmetic covering for the pickup.