A spark plug is disclosed and comprises: a metal tube which interiorly defines an axis and is externally-threaded for engine block engagement; an insulator having a portion which is disposed inside the tube and extends therebeyond; a positive electrode extending through the insulator and projecting beyond the extending portion of the insulator; and an annular ground electrode coupled to the tube. The electrodes are configured such that a spark gap defined therebetween comprises an elongate channel which opens axially and away from said insulator and is substantially unobstructed in the axial direction. The ground defines a void having: a central portion occupied by the positive electrode in use; an annular channel surrounding the central portion; and a plurality of lobes, each being positioned with respect to the central portion in a manner analogous to the placement of the planet gears with respect to the sun gear in a planetary gear.