Core to the Key to Time

Tracers were what Romana I called the core to the Key to Time. They helped the Doctor find 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. Romana usually held the tracer and kept it hidden in her sleeve. (TV: The Ribos Operation) When brought close to disguised segments, it ticked ever more loudly and quickly, as with many ordinary metal detectors. It could also be plugged into the TARDIS console so as to control the TARDIS' navigation, and bring the Doctor to the location of the as-yet-undiscovered pieces of the Key. Once the Key was fully assembled, it could be inserted into the completed Key and, essentially, activate it. (TV: Season 16)

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 Key 2 Time - The Judgement of Isskar)