Part numbers are from Radio Shack. Use caution when enlarging the hole. The actual diameter only needs to accommodate your pole piece (finishing nail). The impedance of the pickup is about 1050 ohms. The idea originally grew out of an attempt to build a hexaphonic pickup for a guitar. The magnet suggested is a small rare earth magnet. I found that small separate magnets gave better separation of the individual pickups. However, if separation is not a concern, a large magnet across the bottom of the pole pieces gives a higher output and causes fewer problems with the magnetic pull on the strings.