A Paradox machine was a device made out of a TARDIS to allow time paradoxes.
History
The Doctor's TARDIS was cannibalized and turned into one by the Master after he stole it from the Doctor. He used it to allow the Toclafane invasion of 2008 Earth.
The paradox machine was destroyed by Captain Jack Harkness with a machine gun. The destruction of the machine reversed time to the point it was activated, undoing everything that had occurred since then. While those within range of the machine on the Valiant remembered the timeline, for the rest of the universe it had never happened. (Template:DW)
- See Year That Never Was.
Physical features
The interior of the paradox machine was lit by an ominous red glow. This may be an indication of the TARDIS's ill health; the dying TARDIS had similar red lighting (Template:DW).
While it was active, the cloister bell alarm system sounded continually.
The main console in particular had gone under many modifications by the Master: The console itself was enclosed inside a metal cage, with various tubes and appendages attached to it. At its base was a pipe connecting to another area of the TARDIS, with a dial showing a readout of the pressure build-up inside. It shot out puffs of steam when it was activated. (Template:DW 2)
Other information
The Doctor's description of the device as a paradox machine indicate that more than one has existed. Jack Harkness also appeared to have some knowledge of a paradox machine, asking the Doctor "Is that what I think it is?" (Template:DW)