Fluid link

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

The First 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)

The Fourth Doctor and Sarah Jane Smith rescued a ship from a pirate fleet of Grakinese Corsairs by exchanging with them some mercury from a fluid link. They got some more mercury in 1931 Chicago, unknowingly defeating the Master's plans by doing so. (VD: The Duke of Dominoes)

When an alien creature broke into the TARDIS, it began leeching the ship of power, affecting the function of the fluid links. (NA: Cat's Cradle: Time's Crucible)


 * The BBC concept art for River Song's Sonic Screwdriver depict a small tubular attachment listed as a "fluid link" along one side of the device, leading up to a section with a blue button, referred to as a TARDIS recall button.