Thermocouplings were components of the TARDIS.

The Fourth Doctor used an astro rectifier and a multi quantiscope to repair the damage done to the thermocouplings after a trip to Kastria. (TV: The Hand of Fear)

Rory Williams caused the TARDIS to materialise inside itself by dropping a thermocoupling when he was distracted by looking up Amy Pond's skirt through the glass floor. (TV: Space / Time)

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