Core to the Key to Time

A tracer — also called the core of the Key to Time — was used to locate the disguised segments of the Key to Time and return them to their original shapes.

Function
The tracer was the core of the Key to Time, in the sense that it slid into the complete cube to furthur tap into its power. (TV: The Armageddon Factor) It would tick to indicate proximity of the segments. When in contact with the tracer, the segments would transform into their original forms. (TV: The Ribos Operation)

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

The first tracer
The tracer first given to the Fourth Doctor and Romana 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))

The second and third 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)

Behind the scenes

 * The sound made by the first tracer resembled that of a Geiger counter.