| 摘要 |
PROBLEM TO BE SOLVED: To minimize the amount of a noble metal used to reduce cost by arranging a chip made of a noble metal joined with an earthed electrode on a facing surface of the earthed electrode facing a center electrode, and forming a fusion zone of an alloy of the constituting materials of the earthed electrode and the chip from the opposite surface to the facing surface toward the chip. SOLUTION: A columnar noble metal chip 52 is insulated from, joined with, fixed to a facing surface 4b of an earthed electrode 4 facing a center electrode held and fixed to a fixing metal fixture at intervals forming a discharge gap. A fusion zone 60 made of the constituting materials of the earthed electrode 4 made of a nickel-base alloy and the chip 52 made of the alloy mainly comprising Ir is formed from an opposite surface 4c to the facing surface 4b of the earthed electrode 4 toward the chip 52. The fusion zone 60 has an alloy of Ni and Ir as the main component, and is thick on the opposite surface 4c side and thin toward the facing surface 4b. The noble metal of the chip equivalent to a part to be arranged in a communicating hole is made unnecessary, and a plug with high productivity and a low price can be obtained.
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