Fluid link

A fluid link is a component of a TARDIS essential for providing power to the ship. Fluid links require mercury to function.

The Doctor falsely claimed that a fluid link had run out of mercury as a ruse to explore the Dalek City. The link was later accidentally left behind in the city, forcing the time travelers to help the Thals fight the Daleks in order to get it back. (DW: The Daleks)

Years later, the fluid links exploded, filling the TARDIS with mercury fumes and forcing the Doctor and Jamie McCrimmon to evacuate the TARDIS. (DW: The Wheel in Space)

The fluid links overheated again when the TARDIS was caught in a volcanic eruption on Dulkis. The Doctor was forced to take the TARDIS out of reality itself. (DW: The Mind Robber)