Rod Coil

A coil wound along the length of a rod-shaped core, usually ferrite, like thread on a spool. Its field runs through the core and comes out of the two ends, the way a bar magnet's does, so it's strongest at the ends and weaker along the sides. Where the ends point decides where the field goes, so in a mat the way the rods are laid sets how much of the field faces the body. The core concentrates the field, which lets a small coil reach a strong peak, but it also raises the coil's inductance, and inductance resists any change in current. So the field rises more slowly than it would without the core, which lowers the slew rate and softens the pulse edge. A ferrite core can also saturate at high drive current. See Air Core vs. Ferrite Core, Coil Inductance (L), Slew Rate (G/µs), Pancake Coil, and Donut Coil.