| 摘要 |
An object of the invention is to provide a technique for improving welding strength between a ground electrode and a noble metal tip. A spark plug includes an insulator having an axial hole extending therethrough in an axial direction; a center electrode provided at a front end portion of the axial hole; a substantially tubular metallic shell which holds the insulator; a ground electrode whose one end is attached to a front end portion of the metallic shell and whose other end faces a front end portion of the center electrode; and a noble metal tip provided on a surface of the ground electrode which faces the front end portion of the center electrode, and forming a spark discharge gap in cooperation with the center electrode. A fusion zone is formed at at least a portion of the boundary between the ground electrode and the noble metal tip through fusion of a portion of the ground electrode and a portion of the noble metal tip. A represents the thickness of the thickest portion of the fusion zone as measured along the axial direction. B represents the length of the longest portion of the fusion zone as measured along the longitudinal direction of the ground electrode. In this case, the spark plug satisfies the relation 1.5 ‰¤ B/A.
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