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