TARDIS magnet
From Tardis Wiki, the free Doctor Who reference
The TARDIS magnet was a pocket-sized device with a green light on it. Using it, one could find their way back to the Doctor's TARDIS.
On the planet Aridius, the Doctor gave Ian Chesterton the TARDIS magnet, so that he could find his way back to the ship in the featureless desert. (DW: The Chase)
