The region of the accessory surface where the magnetic field is strong enough to be useful. It's defined as the area where field strength is at or above 10% of peak. Tissue within this area receives meaningful stimulation. Tissue outside it receives negligible field. The boundary of this area defines the Effective Field Diameter.
Seen from above, a coil lays its field down as a footprint that is strongest over the center and fades with distance. The dashed ring is the usable edge, the distance where the field has fallen to a tenth of its peak. The effective radius reaches from the center to that edge, the effective diameter is twice it, and the shaded disc inside is the effective field area, the surface the coil usefully covers.
