Two distinct metal alloys are joined together to form a thermocouple at a stage called a junction. This junction is called the hot junction or measurement. The leads of the thermocouple are generally connected to an indicator or controller of temperature. This interface is called the reference or cold junction.
A relatively small DC voltage is produced in the thermocouple cables when the measuring junction is heated. The temperature controller measures and transforms the tiny voltage signal to temperature reading. The voltage produced in the thermocouple, however, is so low that milivolts are measured.
Common noise reduction techniques:
Twist and shield the extension or lead cables from the thermocouple to the measuring structure (grounded metal sheath).
Ground the junction at the measurement point. Typically the grounding is to the inside of the sheath of stainless steel covering the real thermocouple. The benefit of grounding the measuring junction is that on each thermocouple wire the electrical noise is spread equally.
Use a very efficient common mode voltage rejection transmitter and locate as close as possible to the thermocouple.