dpb wrote, on Wed, 31 Dec 2014 09:00:35 -0600:

the flame detector is quite a bit more than
just a chunk of SS...
http://www.gayesco.com/Prod_Flame_Rods.htm

PHYSICAL CONFIGURATION
1. The flame rod consists of a center electrode and an outer metallic sheath.
2. The electrode is isolated from the outer sheath by an insulating material.
3. The end of the flame rod, in the flame path, has a special high temperature
hermetic seal with a threaded adaptor which is connected to the center
electrode.
4. A high temperature sensing rod is attached to the threaded adaptor.

I'm confused.
Is the "outer sheath" the ceramic?
Or, is the outer sheath the stainless steel rod exterior?