Structure of matter is determined by electrical interactions between molecules: dipole-dipole, dipole-ion, ion-ion interactions. Impact of an electric field on a matter allows to investigate these interactions and to understand the structure and dynamics of the investigated system. When a dielectric (in this case a liquid) is influenced by an external electric field, it undergoes polarization. For low-intensity field, the polarization is proportional to the field. However, if we increase the intensity of the electric field saturation effects could be expected. The measure of the so-called "non-linear dielectric effect" (NDE) is a nonlinear dielectric increment defined as the difference of the electric permittivity measured in a strong and in a weak electric field intensity:. According to Debye classic theory, in liquids consisted of dipolar molecules, the increment should be negative and proportional to the square of the electric field.
