Patent ID: 8299694
Filing Date: 2012-10-30
Classification: H01T

Abstract:
1. A spark plug comprising: a cylindrical metal shell; an insulator which has a through-hole formed inside the insulator along an axial direction of the metal shell and is held in the metal shell so as to be exposed from the metal shell; a center electrode which is inserted into and fixed to one end portion of the through-hole; a terminal metal piece which is inserted into and fixed to the other end portion of the through-hole; a resistor which is provided between the center electrode and the terminal metal piece in the through-hole and distant from the center electrode in the axial direction; a conductive glass sealing layer which is provided between the resistor and the center electrode in the through-hole without a gap; and a ground electrode which is electrically connected to the metal shell and configured such that a predetermined spark discharge gap is formed between its front end portion and the center electrode, wherein a diameter D of the conductive glass sealing layer is equal to or less than 3.3 mm, a joining surface of the conductive glass sealing layer and the resistor is a curved surface, an assuming that a surface area of the joining surface is Sa and an area of a cross-section of the conductive glass sealing layer, which is perpendicular to the axial direction and includes edge portions of the joining surface, is S