The "Brain in a Jar" is Thaumcraft's means to store experience orbs.  When placed, it will attract and absorb experience orbs with a range of about 8 blocks.  When right-clicked, the jar releases some of the orbs.  It has several limits and drawbacks:

  • It is forbidden research, so both the research, and crafting the jars will inflict some warp.  However, most thaumaturges will at least research it regardless, because the topic is required to learn Infusion Enchanting.
  • The experience is stored only while the jar is placed, and dumped when the jar is picked up.
  • Each jar has a limited capacity, around 2000 XP.  When full, the jar emits green particles, and no longer attracts orbs.
  • The jar casts some light on its surroundings, which is inconvenient for use in some mob farms.