Ceramic heater and glow plug

A ceramic heater extending in an axial direction and adapted to generate heat from its front end portion upon energization, comprising:

• an insulating substrate formed from an insulating ceramic and extending in the axial direction;
• and
• a heat-generating resistor formed from a conductive ceramic and embedded in the insulating substrate, wherein
• the heat-generating resistor includes:
• a heat-generating portion embedded in a front end portion of the insulating substrate, having such a form as to extend frontward from a rear side, change direction, and then again extend rearward, and generating heat upon energization,
• a pair of lead portions connected to respective rear ends of the heat-generating portion and extending rearward in the axial direction, and
• a pair of lead lead-out portions connected to the respective lead portions, extending radially outward, and exposed outward;
• the ceramic heater satisfies an expression S1≦0.34Sa in any cross section of the ceramic heater which is taken perpendicular to the axial direction and in which the lead portions are present, where:
• Sa is a cross-sectional area of the ceramic heater, and
• S1 is a total cross-sectional area of the pair of lead portions;
• wherein a cross section of the ceramic heater which is taken perpendicular to the axial direction assumes a circular form, an elliptical form, or an oblong form;
• in any cross section of the ceramic heater which is taken perpendicular to the axial direction and in which the lead portions are present, of imaginary straight lines which pass through the center of the cross section and along which a gap between the lead portions is measured, an imaginary straight line associated with a minimum gap is defined as a minimum-gap-associated imaginary straight line;
• of intersections of the minimum-gap-associated imaginary straight line and an outline of one of the lead portions, an intersection located on a side toward the center is defined as a point A;
• of intersections of the minimum-gap-associated imaginary straight line and an outline of the other one of the lead portions, an intersection located on a side toward the center is defined as a point E;
• intersections of the outline of the one lead portion and an imaginary circle drawn with the center of the cross section as a center of the imaginary circle and with half of a major diameter of the cross section as a diameter of the imaginary circle are defined as a point B and a point C;
• intersections of the outline of the other lead portion and the imaginary circle are defined as a point F and a point G;
• and
• an angle α formed by a line segment AB and a line segment AC, and an angle β formed by a line segment EF and a line segment EG both range from 160 degrees to 175 degrees.