The Randomiser was a device that would ensure a TARDIS would land at unpredictable times and places.
The Randomiser, when activated, scrambled the TARDIS' co-ordinate settings, giving the Doctor even less control than usual over the destination of his ship. As the name implies, using the Randomiser could send the TARDIS to any point in time and space.
The Randomiser operated by using a seed code to calculate algorithmic impossibilities, making it possible to predict the destination that would be chosen by the Randomiser if others had access to the seed code (PROSE: The Banquo Legacy). When the Time Lords caught up with Compassion, the Doctor attempted to evade them again by using 8.23 as his new seed code based on a bad experience his companion Fitz had with wasps as a child- he was stung twenty-three times after finding a nest when he was eight-, reasoning that it was a number the Time Lords couldn't predict based on their knowledge of him and noting that the seed code simply had to be random rather than complex (PROSE: The Ancestor Cell).
After his successful completion of the quest to find the Key to Time, the Fourth Doctor feared the vengeance of the Black Guardian. Reasoning that if he himself did not know where he was going, neither would the Guardian (and thus the eternal Black Guardian would be unable to pursue or act against him), he installed the randomiser in the central console. (TV: The Armageddon Factor)
- Given the size of the universe and the fact that the TARDIS still showed a marked tendency to materialise on habitable planets, rather than in intergalactic deep space, it appears that the process was not completely random. The Doctor's first two on-screen uses of the device both sent him to planets he had previously visited on numerous occasions (Skaro and Earth).
- Some of the Doctor's statements could be interpreted to mean the Randomiser could be partially engaged, so the Doctor would visit a particular place but at a random point in history - such as in the case of the visit to Paris, where the Doctor complains that 1979 is not a very good year, but then accepts that he can't expect the Randomiser to have very good taste. The implication is that Paris was the Doctor's choice of destination but the actual time period was up to the Randomiser. This may also explain why despite the randomiser, the TARDIS made a second landing in Paris, this time in the year 2000, in COMIC: The Forgotten, which led Romana to question the randomiser's effectiveness.
In the end the Doctor chose to take his chances with the Black Guardian, removing the Randomiser as its components were needed within the Tachyon Recreation Generator on Argolis. He made no effort to replace the device despite his earlier concerns, due to his annoyance to not being able to choose where he wanted to have his adventures. (TV: The Leisure Hive)
Later, in his eighth incarnation, the Doctor found himself being pursued by the Time Lords and was forced to install a second randomiser into the humanoid TARDIS Compassion. While the first randomiser was constructed by the Doctor himself, the second device was purchased on the planet Yquatine. Installing the device caused Compassion great pain, which the Doctor had not anticipated. Possibly due to this traumatic experience, the second randomiser could not be removed or deactivated for some time. (PROSE: The Fall of Yquatine) Compassion was eventually able to expel the remains of the randomiser, along with her occupants, into the Edifice. (PROSE: The Ancestor Cell)
In his tenth incarnation, the Doctor used the randomiser to travel to the 13th moon of the 13th planet of the 13th galaxy. (COMIC: Triskaidekaphobia) He later travelled to Ood-Sphere with Donna Noble, claiming to have set the controls to random. (TV: Planet of the Ood)
- It was not explicitly stated that the Randomiser was used for this; given the Doctor's love of fun and careless nature, he may, over all his years of controlling and upgrading the Tardis, have installed a "Random" button at some point.