The electromagnetic method of investigating the dielectric-metal transition under shock compression of condensed matter is discussed. The investigation of transient electromagnetic processes is performed for the measuring cell comprising two conductive regions (the reference conductor, the test matter). The model E(t) - dependencies of the electric field are found. They can be compared with the corresponding experimental dependencies of the voltage V(t). The E(t) curves are proved to be essentially dependent on the conductivity value and the nature of the conductivity change behind a shock front. The analysis performed provides the basis for the interpretation of the voltage records taken from the experiments with the highly porous aluminium.