In 1925 he discovered that the Debye-Hückel theory of electrolytic solutions was wrong. This theory explained Brownian movement of ions in solution. In 1926 he published a correction to the theory. He traveled to Zürich, where Peter Debye was teaching, and told him his theory was wrong. Debye was so impressed by Onsager's work that he asked him to become his assistant. Onsager stayed at the Eidgenössische Technische Hochschule (ETH) until 1928.