Langevin Theory failed to explain the complicated type of dependence of susceptibility upon temperature exhibited by several paramagnetics,e.g.,compressed and cooled gases,solid salts,crystals etc. Furthermore, theory does not throw any light on the intimate relationship between para and ferromagnetism. Weiss,modified the Langevin theory by introducing a concept of internal molecular field. He assumed that, in a real gas, molecules are mutually influenced by their magnetic moments and consequently there should exist within the gas a molecular field. Taking this internal molecular field under account,Weiss derived the susceptibility of paramagnetic materials,which is called Curie/Weiss Law.
