This circuit controls the exterior shell's Cloaking Device. The Cloaking Device allows the shape, color, mass and texture of the Shell to be altered to blend in with its surroundings. This feature is designed to help prevent changes to history. To supplement the Chameleon Circuit, the telepathic circuits also create a field that makes people passing by the TARDIS more likely not to notice anything odd about it. This field is only effective on people who aren't actively looking for something unusual. If a TARDIS is abandoned by its operator it will deactivate most of its systems, and alter its appearance as necessary to avoid attracting attention. The choice of disguises is nearly infinite (trees, police boxes, chairs, ionized gas...). An Ionic Column disguise comes as a factory preset for a TARDIS.
Usually the operator chooses the disguise, and early types of TARDISes had Chameleon Circuits that had to be manually programmed. Type 58 and later TARDISes can automatically scan the target area right before materialization and select a disguise that will hide it.
Either way, the change can be implemented before or after materialization. If a TARDIS has materialized around an object, the Chameleon Circuit can be set to automatically imitate that object. With careful programming it is possible to make the exterior shell capable of all the functions of the object it is disguised as. It is possible to Time Map the internal dimensions onto the external dimensions. This will increase the Exo-Plasmic Shell to the size of a small moon or planet. However it will limit the capabilities of some of the Defense Mechanisms. If the shell is to have a door then the minimum size is 0.18 cubic meters.
The Shell of a fully operational TARDIS has a very slight vibration that can be detected with the right equipment. However if the Chameleon circuit is faulty, the outer Plasmic Shell might produce an audible hum and easily detectable vibration. Removal of the Visual Stabilization Circuit's fusion plate leaves a TARDIS invisible, but this makes finding the keyhole and the Dimensional Gateway difficult. Older TARDISes were prone to system breakdowns of their Chameleon Circuits, which would freeze the exterior appearance. They required regular Chameleon Conversions to combat this tendency. This problem was fixed with the introduction of the Type 65. If left in the same shape for several years a TARDIS could suffer from chameleonic fluctuation, which causes the exterior to slowly alter its exterior shape and color over time. Each alteration will be minor but the cumulative effect could be quite noticeable. A breakdown between the Basal Programming and the Autonomous Elements will cause a TARDIS to go into Chameleonic Spasm, leading to wild fluctuations in the appearance and size of the exterior shell.
Every time the Chameleon Circuit creates a new exo-shell to disguise the TARDIS a duplicate of the shell is stored in the Shell Room. It should be noted that even if the Chameleon Circuit suffers a complete system breakdown the Shell Room will continue to collect new unused shells every time the TARDIS materializes.
The following color code is used:
- Black: For information from the TV Series, including Dimensions in Time, and 1996 TV Movie.
- Blue: For information from the Novels and Audios including Target, Virgin, BCC, and Big Finish.
- Green: For information from 'licensed' reference sources such as the Technical Manual, Doctor Who Magazine, and the Role Playing Games.
- Red: For information from unofficial sources -The Faction Paradox series, author's speculation, and popular fan belief.
Copyright Will B Swift
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