Re: [EE]: Flame rectification

I have been working on (baffled by) my daughter's Payne high efficiency 
furnace. The flame detector seems to be a simple steel rod. It has only 
one electrical connector with about 90V on it. I assumed, at first, that 
it was some kind of thermocouple referenced to ground through its one 
mounting screw. However, the path from the connector to ground seems to 
be open. Diagnostic instructions indicate that it should carry .5 
micro-amps or more. It looks like the flame provides the path to ground. 
It it gets dirty it doesn't work. Does anybody know anything about this?

Allen Mulvey