Definition
The measuring junction of a thermocouple, placed at the location whose temperature is being measured. Two dissimilar metal wires are joined at this point, and when this junction is heated, a small voltage is generated between it and the cold (reference) junction. That voltage is proportional to the temperature difference between the two junctions and is used to drive a temperature indicator.
Plain English
It is the end of a thermocouple that sits in the hot spot you want to measure, such as a cylinder head or exhaust pipe. The heat at this point is what produces the signal the gauge reads.
Context Anchor
Seen in aircraft temperature-sensing systems, especially engine exhaust gas, turbine, and cylinder temperature indications that use thermocouple probes.
Derivation
A thermocouple has two junctions where the dissimilar wires meet. The one exposed to the heat being measured is called 'hot,' and the reference one kept at a known lower temperature is called 'cold.' The names simply describe which end does which job.
Why Pilots Care
Reliable readings from the hot junction let pilots monitor engine temperatures to avoid overheating and maintain safe operation.
Grounding Statement
The hot junction is the part of the temperature sensor that must actually be in the hot place being measured.
Intuition Check
Do not read “hot junction” as just any warm electrical connection. In this context, it is the specific joined end of a thermocouple that senses temperature.
Example Sentence 1
The mechanic checked that the hot junction of the CHT probe was seated firmly under the spark plug washer before closing the cowling.
Example Sentence 2
A damaged hot junction caused the temperature gauge to read low during the flight.