The bombardment of a glass target by an electron beam has been used to investigate charge propagation in a dielectric medium. A model is described for complex impedance determination. The results are compared with experimental data obtained at room temperature by a capacitance bridge. The validity of the simplified theory of electron and ion propagation which accounts well for variations with electron beam energy is discussed and it appears that measurements by comparison with a standard specimen permits one to ignore the exact value of some parameters whose variation with initial conditions are not known with sufficient accuracy.