Once you get the codes to use the alethiometer you can use it to find secret items and rooms in levels you have already beaten. In fact, it is the only way to get many of the secret items. Also use it to find hidden answers to unlock the meanings to more symbols. For example, when the Master of Jordan college tells you how to find secret answer, if you use the alethiometer it will give you an answer that makes him give you another lore. Also when you are trying to convince Lord Faa to take you along and he gives you the lore, instead of selecting thanks, use find secret answer to unlock a lore. This can also be used on Mrs. Coulter when she asks you to give her the Alethiometer in Bolvanger.