Micromodulator switch

A micromodulator switch was a device capable of shrinking matter, intended mainly for storage. The Doctor swapped his TARDIS's in-flight stereo system for one and installed it in the console, but never got round to testing it and later forgot about it. In his tenth incarnation, he accidentally used it to shrink himself and Rose Tyler, but they managed to get to the switch in their miniaturised condition and throw it in reverse, returning them to normal size. (COMIC: Which Switch?)