The time rotor, sometimes called a time column, was a component in the central column of the TARDIS console. While the TARDIS was in flight, the rotor rose and fell, stopping when the TARDIS reached a destination. It was associated with the 'whooshing' noise heard when the TARDIS was in flight.
The time rotor was connected to the lower engines; hence as the TARDIS moved the rotor moved accordingly. As well as signifying the TARDIS' movement, the rotor was also known to stop working when something went wrong, e.g. the rotor stopped moving as the TARDIS engines stalled. This might be rectified by thumping the console. (TV: Doctor Who)
The Doctor's TARDIS Edit
As with the rest of the Doctor's TARDIS, the aesthetic design of the time rotor occasionally changed throughout the Doctor's travels. As it varied through designs, it alternated between being a single column and a series of components that moved into each other from above and below.
From the start it took the shape of a short, transparent cylinder containing an assortment of components, which rose and fell, with the components circling and rotating within. (TV: An Unearthly Child, et al.)
The Third Doctor's new control console featured a new time rotor with three green inner tubes and a central red one. (TV: The Claws of Axos) Some time during his travels with Sarah Jane Smith, the Fourth Doctor's time rotor had its green inner tubes replaced with red ones. (TV: Planet of Evil) He occasionally placed his hat atop of it. (TV: The Stones of Blood, The Horns of Nimon)
The Fifth Doctor later gained a new console with a new time rotor. (TV: The Five Doctors) At one point during the Sixth Doctor's travels, a Whifferdill named Frobisher assumed the form of the time rotor. (COMIC: The Shape Shifter)
After the TARDIS console was once again reconfigured due to the Doctor's ring interfacing with the chameleon circuit icon system, the time rotor became a cross between its previous design and its original. (COMIC: The Chameleon Factor)
Some time during the Seventh Doctor's life, the TARDIS console room and console were reconfigured once again. In this design, the time rotor was a tall, transparent tube which attached to the ceiling as well as the console. Inside were a pair of glowing, opaque, blue-white tube assemblies which would rise and fall in alternation during flight, meshing and unmeshing in the centre. (TV: Doctor Who)
The War, Ninth and Tenth Doctors' time rotor also reached the ceiling, and contained a set of glowing, transparent inner assembly of tubes, which glowed both white (TV: The Day of the Doctor) and later turquoise. (TV: Rose)
When that console room was destroyed and a new one regenerated by the TARDIS, the Eleventh Doctor was greeted by a time rotor which contained transparent, bulbous inner components resembling blown glass. (TV: The Eleventh Hour)
After the Doctor became seclusive following the departure of the Ponds, the TARDIS interior regenerated once again. Because of this change, a new console room and console were introduced, and with them a new time rotor, composed of a hexagonal array of neon turquoise tubes that connected the ceiling to the console (as its previous incarnations). Though while in contrast the newer rotor appeared to be fixed, lights inside the tubes rhythmically flashed in sequence when in flight, stimulate the 'pumping' motion of the temporal piston. There were also two small crystal-like prisms in the top and bottom centres of the the time rotor that physically pumped up and down. (TV: The Snowmen) Once he had regenerated, the Twelfth Doctor changed the colouring of the turqoise pipes in the time rotor to a warmer orange. (TV: Deep Breath)
After the TARDIS malfunctioned following the Twelfth Doctor's regeneration, it chose a new console room to replace the one he destroyed. In this latest iteration of the interior, the time rotor appeared as a giant crystal. (TV: The Ghost Monument). Notably, the time rotor does not reach up to the ceiling and instead undulates independently in the traditional manner the TARDIS did before used by the Seventh Doctor near the end of his life. (TV: Rosa)
Other TARDISes Edit
The Junk TARDIS built by the Eleventh Doctor and Idris featured a short time rotor salvaged from a dead TARDIS, plugged into a similarly salvaged console. It contained lights and metal components. It glowed pink when activated with a touch by Idris. (TV: The Doctor's Wife)