Atom accelerator
From Tardis Wiki, the free Doctor Who reference
An atom accelerator was a TARDIS component. In the Eleventh Doctor's TARDIS, it was a spiky ball on the navigation panel that spun while in flight and during take-off. According to the Eleventh Doctor, it steered the TARDIS. Henry Avery compared it to a wheel on his vessel, the Fancy (DW: The Curse of the Black Spot)
