The golden compass is also an item within the story.
Spoilers below

The golden compass, properly called an alethiometer (from greek for truth and meter), is a dial with three hands and a face divided into thirty-seven sectors. Each sector bears a symbol. The user holds a question in mind, and turns the hands to the symbols that encode the content of the question. He or she then settles into an attentive, trance-like state, and the hands rapidly spin around and settle. Their movements and ultimate position can be interpreted, through laborious research, as a list of terms. The user must then use intuition and lateral thinking to understand the implied linkages between the words. When the alethiometer is read properly, it always speaks truly and can answer any question. The only limit is the knowledge and wisdom of the interpreter. Its answers are sometimes typically oracular ("you will face a great betrayal"), and sometimes thoroughly prosaic ("look in the third tree from your left").

In His Dark Materials, Lyra discovers that she has an inborn ability to read the alethiometer without training.

Although the full range and technique of alethiometry is not explicated in the novel, Pullman (or possibly a fan working for his publisher) has put a more complete explication on the book's website (http://www.randomhouse.com/features/pullman/goldencompass/).

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