Laser beam is often used in the research of electro-dynamic and pre-breakdown processes in liquid dielectrics. Non-linear optic effects, caused by heating of a liquid, can appear even when a laser beam of a relatively small power 5-10 mW passes through a medium. Certain non-homogeneous temperature distribution and consequently refraction coefficient distribution can be observed in the region through which a laser beam passes. The above-mentioned region produces a defocusing influence on a liquid. Thus, for example, when a helium-neon laser (the power 10 mW) passes through a 10 cm layer of transformer oil, the angle of defocusing reaches 10^-3 rad.