The measured impedance of vitreous materials plotted in the complex plane permits the representation of their electrical properties by an analytical equation of this impedance as a function of frequency and three characteristic parameters which can be determined with accuracy.

Electrical properties of the same glasses under the same experimental conditions have already been studied and in this paper the value of impedance diagrams as compared to the usual representations of experimental results is studied. These results are interpreted by comparing the material to a simple parallel combination of a capacitor and a pure resistance.

It is shown that such an arbitrary equivalent circuit gives a complex permittivity whose frequency dependence, in the case of ionic conductors, cannot be deduced with accuracy from the experimental data. In particular it is shown that the existence of a low frequency relaxation deduced from these conventional representations is entirely dependent on the accuracy of measurements.