The possibility of determining the electron density using the Faraday effect is discussed. The regions in the density-frequency diagram which may be used for the experimental determination of the electron density are restricted by the following two conditions; (1) Dielectric constant of the system in the magnetic field to be positive.
(2) The rotation angle is large enough to be measured. The possible regions are shown in graphs as the function of the electron density and the magnetic field for several frequencies.