In this work present an approximate calculation of the magnitude of the magnetic induction vector of five active galactic nucleus (AGN) using the Blandford - Znajek mechanism. It is assumed that power associated with the Blandford - Znajek mechanism is comparable with the luminosity of the H alpha line of the AGN. In consequence, the corresponding spectra are analyzed computationally to determine the energy flow of the H alpha line and the redshift, magnitudes on which this luminosity depends. The extinction of the H alpha emission line, visible extinction and redness, which are also required, are calculated using online operators of the NASA IPAC extragalactic database. Finally, the results are analyzed with the purpose of trying to determine the role of the magnetic field in the stability of the region surrounding the supermassive central object.