The aim of this study is the measurement of the ferrofluids Faraday effect, with a low magnetic field applied (between 0 and 500 G) and various volumic magnetic concentrations. For each of them, we measured the value of the minimum longitudinal magnetic field applied, driving to a self developing phase transition (appearance of clusters). This leads to lessen or cancel the transmitted light beam. A dynamic study of this effect has been made (by switching on and off the magnetic field applied to the ferrofluid) that carried out a large asymmetry.