Definition
In a thermocouple-type temperature measuring system, the cold junction is the reference junction held at a known, lower temperature against which the temperature at the hot (measuring) junction is compared. The voltage produced by a thermocouple depends on the temperature difference between its two junctions, so the cold junction provides the stable baseline that allows the instrument to translate that voltage into an accurate temperature reading.
Plain English
It is the cooler reference end of a temperature sensor that uses two joined dissimilar metals. The instrument measures how much hotter the other end is compared to this reference end, and that difference is shown as the temperature reading.
Context Anchor
Seen in aircraft temperature indicating systems, especially when discussing thermocouple-based engine temperature instruments.
Derivation
From the original design of thermocouple circuits, where one junction was kept literally cold (often in an ice bath) to provide a stable reference, while the other junction was placed at the hot spot being measured. The names stuck even though modern systems use electronic compensation instead of an actual cold bath.
Why Pilots Care
Stable cold-junction compensation is required for reliable engine temperature indications; drift or failure here produces false readings that can mask overheating or other engine problems.
Grounding Statement
A thermocouple system measures temperature by comparing a hot sensing point with a cooler reference point.
Intuition Check
Do not read “cold” as meaning frozen or even necessarily cold to the touch. Here it means the reference junction that is cooler than the hot measuring junction, or whose temperature is known and compensated for.
Example Sentence 1
The CHT gauge compares the temperature at the cylinder against the cold junction inside the instrument to display an accurate reading.
Example Sentence 2
During preflight the pilot noted that the temperature indicator automatically compensated for changes at the cold junction.