In most cases people mix up between coil and spiral. Coils and spirals are two closely related terms and ideas. We interchange these two terms quite frequently. Isn’t it we usually say ‘snake coil’ and ‘spiral’ staircase? For the design purpose, though, we will definea coil as a series of loops of material that is of more or less constant radius (plate 11A&B) as when wrapped around a cylindrical core, whereas a spiral is made of loops that that steadily increase in diameter as they move outward from the center (plate 11 C&D). A coil is more or less synonymous with a helix. If a coil is attached to a core, as are the threads on a bolt, the term ‘helix’ is frequently used. Interestingly we do not realize the basic difference in the paradigms of ‘coil’ and ‘spiral’.
