Core to the Key to Time

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)

Tracers tracked the segments by the chronon radiation they would give off. This meant that any other form of radiation registered as a possible segment. (PROSE: The Shadow of Weng-Chiang)

There were three known tracers. The first was an inanimate object, while the other two were living beings.

Original tracer
The tracer first given to the Fourth Doctor and Romana I by the White Guardian had the form of a short silver wand with pink lights. 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)

The tracer was also used to disperse a segment of the Key to prevent it from being used by Millicent Ferril. (AUDIO: Ferril's Folly).

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
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)