US Patent 7595714 Ignition coil

An ignition coil includes a primary coil (14), a secondary coil (16) disposed on an outer circumferential side of the primary coil and configured to be boosted by mutual induction with the primary coil, an outer periphery core (18) having an opposing surface (183), which is opposed to an outer peripheral surface (160) of the secondary coil, and an insulating member (20) disposed between the outer peripheral surface and the opposing surface. The secondary coil and the outer periphery core are arranged such that a shortest distance between the outer peripheral surface and an outer edge (183a, 183b) of the opposing surface is larger than a shortest distance between the outer peripheral surface and the opposing surface.