Abstract: A light emitting element of one or more embodiments includes a first electrode, a second electrode oppositely to the first electrode, and an emission layer between the first electrode and the second electrode. The light emitting element of one or more embodiments includes a polycyclic compound represented by a specific chemical structure in the emission layer, thereby showing improved emission efficiency and life characteristics.

Abstract: A spark plug having a metallic shell covered with a nickel plating layer and having a groove portion formed between a tool engagement portion and a gas seal portion and having an orthogonal-to-axis sectional area of 36 mm2 or less.

Abstract: A spark plug includes an insulator extending along an axis and a metal shell provided on the outer periphery of the insulator. The metal shell includes a thread portion for threadedly engaging with a mounting hole of an internal combustion engine and a seat portion having a tapered surface whose outer diameter decreases gradually toward a leading end side. The tapered surface contacts with a bearing surface of the internal combustion engine when the thread portion is engaged with the mounting hole. Annular protruding portions or a helical protruding portion are formed within a range on the tapered surface from an outermost peripheral portion to a part forming an outer diameter occupying 95% of the diameter of the outermost peripheral portion. The arithmetic mean roughness of a surface of the tapered surface within the range is set to 1-5 ?m on a section including the axis.

Abstract: A spark plug includes an insulator extending along an axis and a metal shell provided on the outer periphery of the insulator. The metal shell includes a thread portion for threadedly engaging with a mounting hole of an internal combustion engine and a seat portion having a tapered surface whose outer diameter decreases gradually toward a leading end side. The tapered surface contacts with a bearing surface of the internal combustion engine when the thread portion is engaged with the mounting hole. Annular protruding portions or a helical protruding portion are formed within a range on the tapered surface from an outermost peripheral portion to a part forming an outer diameter occupying 95% of the diameter of the outermost peripheral portion. The arithmetic mean roughness of a surface of the tapered surface within the range is set to 1-5 ?m on a section including the axis.

Abstract: A spark plug having a metallic shell covered with a nickel plating layer and having a groove portion formed between a tool engagement portion and a gas seal portion and having an orthogonal-to-axis sectional area of 36 mm2 or less.