<span class="SFPTagline"> From SCIFIPEDIA </span>

On the long running series, Doctor Who, the Time Lords use a device known as a chameleon circuit to disguise their time travel vehicles. Each of these vehicles, commonly known as a TARDIS, comes equipped with such a device. During the materialization of a TARDIS in a new location, the chameleon circuit scans the local environment and chooses an external appearance for the TARDIS that will hopefully allow its presence to remain undetected. The external shape and features of a TARDIS appear to be almost infinitely variable. They have been seen in shapes ranging from a giant rocketship to a rather small china cabinet. The display screens in the Doctor's TARDIS have also suggested that shapes such as palm trees and even the great pyramid are possible.

When the Doctor's TARDIS materialized in 1963 London (in 1963's "Un Unearthly Child"), his chameleon circuit chose the shape of a Police Box, a blue box, roughly the size and shape of a phone booth, that could be used to make emergency calls. This was a common site on the streets of London at the time, though there are few left these days. It would have been an appropriate disguse at the time except that it materialized in the middle of a junkyard, hardly a likely place for a public emergency phone. This may have been a sign that the Doctor's chameleon circuit was malfunctioning. It ceased to function entirely shortly thereafter, locking the Doctor's TARDIS in the police box form for most of his adventures to date.

The nature of the failure in the Doctor's chameleon circuit required the use of block transfer computation to repair. According to the 1980 story "Logopolis," this particular branch of mathematics (which the Logopolitans specialized in) had the power, when performed by living individuals rather than machines, to affect the very fabric of the universe. However, it appears that this was not the only step required in the repair. When the Doctor left Logopolis, his TARDIS remained a police box.

It wasn't until 1985's "Attack of the Cybermen" that the Doctor finally completed the repairs. His TARDIS was seen to change into several different forms during the course of that story until finally returning to the shape of the familiar blue box. At that time, the Doctor simply concluded that his TARDIS didn't want to change. He has not attempted to tamper with his chameleon circuit since.