Isomorphic controls

Isomorphic controls could only be operated by one user. Such controls ostensibly worked only after identifying the allowed user through genetics or other uniquely identifying properties.

One such device was the Master's laser screwdriver. The Doctor attempted to use it to overpower the Master, only to be ridiculed when he could not. (DW: Last of the Time Lords) At one point, the Doctor's TARDIS key was also said to be isomorphic, (DW: Spearhead from Space) but other, non-isomorphic security features were posited. (DW: The Daleks) Also, the fact that the Sixth Doctor could gain access to other TARDISes using his own TARDIS key seemed to argue against the keys being strictly isomorphic, (DW: The Mark of the Rani) as did the fact that relative strangers like Ben and Polly could effect entry into the Doctor's TARDIS by means of keys not especially made for them. (DW: The War Machines)

Like its keys, the TARDIS' controls were said to be flatly isomorphic, (DW: Pyramids of Mars) or at least uniquely operable by the Doctor. (DW: The Daleks) This fact prevented a shape thief from stealing the TARDIS, even though he had perfectly mimicked the first incarnation of the Doctor. (CC: Mother Russia) However, on another occasion, the Doctor's fifth self indicated that the existence of isomorphic controls was only a bluff. (BFA: The Bride of Peladon) Indeed, there were several instances of companions successfully using — or even being actively taught to use — the TARDIS controls. (DW: The Pirate Planet, Castrovalva, Four to Doomsday, Journey's End, The Time of Angels, The Pandorica Opens) It was unknown whether this meant that the isomorphic security lockout ceased to function over time, that the Doctor could temporarily disable it, or even that the TARDIS simply granted access to companions, in the same way it did to her translation circuit (a possible example being when the TARDIS consciousness instructed Rory Williams on how to lower the TARDIS shields from an older control room). (DW: The Doctor's Wife)

Kazran Sardick's machine had isomorphic controls. After the Doctor changed his personality, they no longer recognised him. (DW: A Christmas Carol)

Gwen Cooper lied to Rex Matheson that the Eye-5 contact lenses were isomorphic. (TW: Dead of Night)