TARDIS magnet

The TARDIS magnet was a pocket-sized device with a green light on it. Using it, people could find their way back to the Doctor's TARDIS.

In a featureless desert on the planet Aridius, the First Doctor gave his companion Ian Chesterton the TARDIS magnet to ensure that he and Vicki could find their way back to the ship after exploring. (TV: The Chase)

A similar device was in the form of a wristwatch worn by the Third Doctor in 1970. It allowed him to find UNIT's secret headquarters and the TARDIS therein. (TV: Spearhead from Space)