- "It was very like a clock, or a compass, for there were hands pointing to places around the dial, but instead of the hours or the points of the compass there were several little pictures, each of them painted with the finest and slenderest sable brush."
- —Description of the alethiometer[src]
An Alethiometer was a small, gold-coloured, compass-like device that had a single, simple, use: to tell the truth. Its name is Aletheia "truth" + -ometer "measuring device". An alethiometer was given to Lyra Belacqua by the master of Jordan College, who told her to keep it a secret from Mrs. Coulter.
A particular process was to be followed to use an alethiometer. The user directed three needles to lie over certain symbols on the face of device to create a question. Once such an alignment of the needles was achieved, the user had to define or think of the question in his / her mind. The user must not grasp for an answer, but be content not to know. Once the question was formed lightly in the mind of the user, the fourth needle swung into action and moved from one symbol to another. It may point to one symbol more than once, specifying the meaning. Any particular symbol had multiple meanings and answers. The user had to learn to decipher the answers and it took much experience to be able to do so. An Alethiometer provided the answers through the influence of Dust (dark matter and dark energy) or particles called Shadows. An alethiometer could provide answers to past and present events, but it couldn't foretell future events. However, it may predict in a limited way about future events, as to what would happen if certain actions were taken; for instance, the effects of leaving the underworld on the ghosts that later did so in The Amber Spyglass.
A person who had mastered the art and science of reading and understanding the alethiometer was called an alethiometrist. Notable alethiometrists included The Master of Jordan College, Fra Pravel and Lyra Belacqua. Lyra was especially notable because she could read the alethiometer with relative ease using her intuition. Most readers required years of study and volumes of reference books, and still took a very long time to make readings.
The 36 symbols which appeared on an Alethiometer and some of their meanings with pictorial presentations are given below: