Spark plug with noble metal tip

1. A spark plug comprising:

a center electrode extending in a direction of an axis;

a ceramic insulator having an axial bore extending in the direction of the axis and holding the center electrode in the axial bore;

a metallic shell provided externally of an outer circumference of the ceramic insulator; and

a ground electrode having an extending portion extending in the direction of the axis, attached to the metallic shell at one end, and forming, at the other end, a gap in cooperation with the center electrode, at least one of the center electrode and the ground electrode has an electrode base metal;

a columnar noble metal tip disposed in such a manner as to face the other electrode; and

an intermediate member disposed between the electrode base metal and the noble metal tip, said intermediate member having a first surface and a second surface the first surface of said intermediate member being in contact with the noble metal tip and the second surface of the intermediate member being in contact with the electrode base metal, said second surface of said intermediate member having an area greater than that of a cross section of the noble metal tip defined by a plane cut parallel to the first surface;

said intermediate member attached to said electrode base member by a nugget that is formed through fusion and solidification resulting from welding at least a portion of the boundary between the intermediate member and the electrode base metal; and

in a section of the spark plug cut by a plane which passes through the center of gravity of the intermediate member and is in parallel with a facing direction between the extending portion and the center electrode, a relational expression S1/(D1×H1)≧0.005 is satisfied, where

S1 is a total area of the nugget,

H1 is a height of an end surface of the noble metal tip from a disposition surface of the electrode base metal where the intermediate member is disposed, and

D1 is a maximum width of the noble metal tip.

2. A spark plug according to claim 1 , wherein

when the noble metal tip and the nugget are projected vertically onto a plane parallel with the disposition surface, the nugget includes a portion located inside an outline of the projected noble metal tip.

3. A spark plug according to claim 1 or 2 , wherein

in the section,

the nugget is formed on opposite sides of a centerline which passes through the center of gravity and is perpendicular to the end surface.

4. A spark plug according to claim 3 , wherein

in the section,

the nugget is further formed at a location on the centerline.

5. A spark plug according to claim 1 or 2 , wherein

a relational expression S1/(D1×H1)≧0.029 is satisfied.

6. A spark plug according to claim 2 , wherein

in the vertical projection, the nugget further includes a portion located outside the outline of the projected noble metal tip.

7. A spark plug according to claim 6 , wherein

the intermediate member has an attachment portion greater in an area of a cross section cut by a plane parallel to the first surface than the noble metal tip;

the attachment portion ranges from the disposition surface to a height of 0.2×H1; and

in the section,

a relational expression S3/(H2×D2)≧0.030 is satisfied, where

H2 is a height of the attachment portion from the disposition surface,

D2 is a maximum width of the attachment portion, and

S3 is a total area of those portions of the nugget which are formed between the attachment portion and the electrode base metal and are located outside the range of the noble metal tip with respect to the width direction of the attachment portion.

8. A spark plug according to claim 1 or 2 , wherein

the nugget is formed internally in such a manner as to not be exposed at an outer surface of the intermediate member, and

in the section,

a relational expression L1≧0.10 mm is satisfied, where

L1 is a shortest distance between the nugget and outlines of the intermediate member and the noble metal tip.

9. A spark plug according to claim 1 or 2 , wherein

in the section,

the nugget is formed along the overall boundary between the electrode base metal and the intermediate member.