Core to the Key to Time

A tracer acted as a means to locate the diguised segments of the Key to Time and to transmute them back into original shapes.

The first tracer
The tracer first given to the Doctor and Romana by the White Guardian had the form of a short silver wand with pink lights. (DW: The Ribos Operation) With five of the six segments completed, the tracer slid into the mostly completed core of the Key to Time through a hollow tube. (DW: The Armageddon Factor)

The second and third tracers
Once more sent on a mission to recover the segments, the 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. (BFA: The Key 2 Time - The Judgement Of Issak)