The slugs or studs as they are commonly called are made from a soft iron and are used to conduct the magnetic field from the bar magnet to the strings. The studs are 3/16″ in diameter and are pressed into the non-adjustable bobbin often concealed by a pickup cover. The corners of the studs have a slight chamfer for ease in assembly as they are pressed into the bobbin. The studs are normally flush or flat with the top of the bobbin and extend out from the bottom of the bobbin. They usually extend the thickness of the bar magnet and conduct the desired magnet polarity to the strings.