The electrostatic attraction between ions in solutions of electrolytes that causes them to associate in pairs. As a result, complete dissociation does not take place and experimentally found electrical conductivities are lower than those predicted by the Debye–Hückel theory. In 1926, Neils Bjerrum (1879–1958) proposed improving the Debye–Hückel theory by taking ion association into account. He found that the significance of ion association increases as the relative permittivity of the solvent decreases.