Resistor built-in spark plug

A resistor built-in spark plug comprising an insulator formed with an axially extending inner hole, a stem fitted in the upper part of said hole in said insulator, a center electrode fitted in the lower part of said hole, a resistor disposed in the central part of said hole, a first copper-glass electrode layer disposed between said resistor and said stem and in contact with said stem, a second copper-glass electrode layer disposed in contact with said resistor and containing a greater amount of copper than said first copper-glass layer, and a third copper-glass electrode layer disposed between said resistor and said center electrode, said resistor being made by firing and solidifying powder of resistor mixture consisting of 75 to 38 volume percent of main resistor component containing tin oxide and 25 to 62 volume percent of glass powder with softening temperature of 300.degree. to 600.degree.C, said first and third copper-glass layers being made by firing and solidifying a mixture of 10 to 35 volume percent of copper powder and 90 to 65 volume percent of glass powder with softening temperature of above 530.degree.C and more than 30.degree.C higher than the softening point of glass in the resistor portion, and said second copper-glass layer being made by firing and solidifying a mixture of copper powder in an amount of more than 30 volume percent and greater than the amount of copper in said first copper-glass layer and remaining percent of glasss powder with softening temperature of above 530.degree.C and more than 30.degree.C higher than the softening temperature of glass in the resistor portion.

A resistor built-in spark plug comprising an insulator formed with an axially extending inner hole, a stem fitted in the upper part of said hole in said insulator, a center electrode fitted in the lower part of said hole, a resistor disposed in the central part of said hole, a first copper-glass electrode layer disposed between said resistor and said stem and in contact with said stem, a second copper-glass electrode layer disposed in contact with said resistor and containing a greater amount of copper than said first copper-glass layer, a third copper-glass electrode layer disposed between said resistor and said center electrode and in contact with said center electrode, and a fourth copper-glass electrode layer disposed in contact with said resistor and containing a greater amount of copper than said third copper-glass layer, said resistor being made by firing and solidifying powder of resistor mixture consisting of 75 to 38 volume percent of main resistor component containing tin oxide and 25 to 62 volume percent of glass powder with softening temperature of 300.degree. to 600.degree.C, said first and third copper-glass layers being made by firing and solidifying a mixture of 10 to 35 volume percent of copper powder and 90 to 65 volume percent of glass powder with softening temperature of above 530.degree.C and more than 30.degree.C higher than the softening point of glass in the resistor portion, and said second and fourth copper-glass layers being made by firing and solidifying a mixture comprising more than 30 volume percent of copper powder greater in amount than copper in said first and third copper-glass layers and remaining percent of glass powder with softening temperature of above 530.degree.C and more than 30.degree.C higher than the softening point of glass in the resistor portion.