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The core to the Key to Time, or tracer, helped the Doctor find and transmute the individual, disguised segments of the Key to Time, and then served as the activator for the completed Key. (TV: Season 16)
There were three known tracers. The first was an inanimate object, while the other two were living beings.
Original tracer Edit
It had two main locational abilities. Plugged into the TARDIS console, it revealed the space-time co-ordinates for the planet on which an as-yet-undiscovered segment resided. Once the Doctor landed on a planet, it was then disengaged from the console and carried, usually by Romana. Once brought close to a segment, it ticked ever more loudly and quickly, much like a Geiger counter.
When brought into direct contact with a disguised segment, it could then transform the object into its true form — one of several pieces of a cube. After the Key was fully assembled, it could be inserted into the completed Key and, essentially, activate it. (TV: Season 16)
Ultimately, the Fourth Doctor snapped the tracer in half to prevent the Black Guardian from using the Key. This act scattered the Key to Time throughout the universe, and returned Astra of Atrios to her human form. (TV: The Armageddon Factor)
Human tracers Edit
Once more sent on a mission to recover the segments, the Fifth Doctor had as his companion, Amy, a tracer in the form of a human. Amy had a "twin sister", Zara, also a tracer in human form. (AUDIO: The Judgement of Isskar)