| 摘要 |
To prevent a heating element from lodging or deforming. Sloping tapers 40b and 40c are provided to a side wall of a mounting groove 40 with a heating element 42 in a way that they recede from the heating element 42, as the element advances to the center of the heat insulating wall 33. When the heating element 42 moves in an externally radial direction within the mounting groove 40 depending on heat expansion, it is possible to prevent the heating element 42 from hooking into the tapers 40b and 40c, so that the heating element 42 can return to the previous position by moving in an internally radial direction within the groove 40, even if the heating element 42 shrinks for temperature fall. Heating element disconnection due to heat expansion or shrinkage in the heating element can be avoided by preventing changes for heat expansion or heat shrinkage in the heating element, and consequently the life cycle of the heating element extends.
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