Comparator

A comparator was an essential component in a TARDIS. Peri Brown took the comparator from the Doctor's TARDIS and kept it so could not gain control of the TARDIS. She entrusted it to Vislor Turlough, who returned it to the Fifth Doctor. (TV: Planet of Fire) Later, when the TARDIS experienced a power failure, Peri asked the Sixth Doctor if the problem was with the comparator. (TV: Vengeance on Varos)