Relative Dimensional Stabiliser

The dimensional stabiliser of a TARDIS (specifically one of the kind used by Mortimus) regulates the size of the interior of the ship in relation to its exterior. By sabotaging Mortimus' dimensional stabliser, the Doctor made the inside of Mortimus' TARDIS the same size as the outside which meant that Mortimus could not squeeze in through the doors of his ship. (DW: The Time Meddler)