Sentence:  This arrangement had the purpose to prevent heated gas to reach the <WSD>thermocouple</WSD> by natural convection . 
Meaning of the word marked with <WSD> tags:  a kind of thermometer consisting of two wires of different metals that are joined at both ends; one junction is at the temperature to be measured and the other is held at a fixed lower temperature; the current generated in the circuit is proportional to the temperature difference