[Cob] How to measure heat inside oven or furnace?

[Peter Kaulback]

I'm almost done my oven and am considering a rocket stove for a 
livestock area, heat is precious at lambing especially if it's damp :(

I've always wondered how one might determine the temperature inside an 
oven or furnace, is there a special thermometer one can use?

Peter Kaulback
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I haven't failed, I've found 10,000 ways that don't work.

Thomas Edison (1847-1931)