Exterior

Exterior (外装代脳(エクステリア)) is a large cultivated brain that greatly boosts Shokuhou Misaki's power. Originally, used as a means for other individuals to use Mental Out, it is later taken over by Misaki for her personal use. It is later destroyed during her struggle against Kihara Gensei after he took it over for his nefarious Level 6 Shift attempt on Misaka Mikoto.

Principles
The giant brain was originally intended in the Exterior Project as usable by anyone, whether they are other espers or normal people, any registered person with Exterior gains the ability to use Mental Out. Registration for its usage apparently takes several days, and it is unknown what the requirements are. There is also a limiter for the Exterior likely to prevent it from being used to its maximum capacity. A code is required to disable it that is only known to Misaki. There is also a self-destruct code which can be used to destroy the brain.

The self-destruct code as seen in the manga is an string that goes as follows: LnuVAF-BXesGDVjLnzNN-2SVgsPYhchf-mY7XAKR85RSCj-DtydrVMYDrMz9-yZQL4LZgMcxGA-X3MwXgYjBp9A4-JGdc4c

Under the control of Misaki it gives her a tremendous boost of her powers. After speaking some commands to it, apparently regardless of distance, she can use her Mental Out from several hundred meters range and absolutely controlling a maximum of 10 people (or from about 100 people as long as they are doing simple commands) to a range of several kilometers, allowing her to put thousands of people under her control.

Exterior is designed to withstand five times the power of the greatest earthquake recorded, and requires the self-destruct code in order to be destroyed safely, as damaging the brain risks crippling the mind of the user.

Exterior is located in a facility in School District 2, formerly used for the Clone Dolly Workshop.

Security
The key to defending Exterior is it being concealed hence the reason why Misaki goes out of her way to maintain its secrecy. However, if this discovered Misaki has some countermeasures against intruders inside the building that houses it such as surveillance cameras, thick metal doors, electrified floors, and flamethrowers, all under the control of Misaki. However, there are other unique technology found in the building:
 * Hag's Hug (魔女の抱擁(ハッグハグ) Toaru_Kagaku_no_Railgun_T_E12_18m_05s.png): When activated, specifically when it is stepped upon, it emits an oscillatory wave when it enters the body through the foot. It is then reflected and amplified within the body as it travels through it and ultimately converges together. It destroys the victim's inner ear and shakes their brain.
 * Graviton Panel (重力子寄木板</rb>(</rp>グラビトンパネル</rt>)</rp> Toaru_Kagaku_no_Railgun_T_E12_18m_45s.png): Is a small metallic panel that when combined with other panels can form a sturdy solid surface. They can form floors, bridges, and walls. According to Misaki they were originally made to create passageways as so she can use shortcuts, as well as temporary scaffolding during construction. It is under the complete control of Misaki.

Background
After discovering Misaki's Mental Out ability, the researchers of the Clone Dolly Workshop decided to use Misaki's powers to brainwash great men, instead of creating them.

To create Exterior, the researchers take a portion of Misaki's cerebral cortex, cultivate it, and enlarge it. They cut it in half between the two sides and put the two pieces into two glass tubes with liquid in it.

Prior to Exterior's full completion, Misaki's friend Dolly dies, and here, Misaki learns of the true nature of the project, after taking control of the minds of the researchers, who hoped to get rid of her once Exterior is complete She later has the project crushed completely, brainwashes everyone involved with it, and takes it over for her own purposes, apparently filling it with people loyal to her in order to perform maintenance on it.

Daihasei Festival Arc
Misaki can apparently issue Exterior commands in order to greatly increase her mind control abilities. In chapter 59 of the Railgun manga when this is first seen, Misaka Mikoto, who was with Misaki at the time, thinks that the powers granted to Misaki by Exterior far surpass the abilities that a level five should be able to use. It seems as if Shokuhou does not like to rely on the use of Exterior. Moreover, during her command, a small panel of eyes overlayed upon a background filled with liquid and tubes are briefly shown, indicating as a visual representation that it is at least human as the eyes displayed the properties of being controlled by Misaki.

It is later used and taken over by Kihara Gensei in order to facilitate the making of Misaka Mikoto into a Level 6, although he could not use it completely because of the limiter Misaki has on it. Gensei pursued Misaki through the facility in order to obtain Exterior's limiter release code. Despite making use of the various security measures, Misaki was eventually cornered and had the code taken from her. Seeing Mikoto's progress, Gensei entered the code straight away. However Misaki had previously swapped the limiter release code in her memory with the self-destruct code, resulting in Exterior's destruction.

Agitate Halation Arc
At some point during this arc, when Misaki reveals to Touma the extent of her powers, she makes it clear that she does not have the increased abilities that Exterior allows her to use. She does not explicitly mention Exterior at all.

Trivia

 * The name of the countermeasure "Hag's Hug" might be a reference to the Old Hag or, a creature from English folklore which would sit on a person's chest while they were sleeping, causing shortness of breath and paralysis.