Spark plug and internal combustion engine equipped with the spark plug

An internal combustion engine comprising:

• a cylinder including a combustion chamber and a spark plug attached to the cylinder, said spark plug having a spark discharging end projecting into said combustion chamber;
• said spark plug comprising:
• a center electrode containing nickel;
• an insulator having an axial hole extending along an axial direction of the center electrode, and holding the center electrode in the axial hole;
• a metallic shell radially surrounding and holding the insulator;
• a ground electrode containing nickel as a main component and having a first end portion joined to the metallic shell and a second end portion facing the center electrode;
• and
• a noble-metal chip containing platinum or iridium as a main component and joined to the second end portion of the ground electrode;
• the spark plug generating spark discharge between the center electrode and the noble-metal chip;
• wherein the nickel content of the center electrode is greater than the nickel content of the ground electrode;
• and
• said internal combustion engine further comprising means for applying a discharge voltage to the spark plug such that the center electrode of the spark plug assumes a positive polarity and the ground electrode assumes a negative polarity.