The study of the properties of a metal-dielectric-metal system in a strong electric field has so far led to heterogeneous results. Their agreement with theory is unsatisfactory or lacking entirely. The theoretical conceptions are based on the assumption of the field emission of electrons from the surface of metals to the conduction band of the dielectric and from there by passage through the second metal electrode to vacuum. A perfect dielectric is assumed here. This assumption is probably not fulfilled when realizing the cathodes, as can be estimated from a comparison of the technology of producing a dielectric, for example, with the preparation of intrinsic semi-conductors. The presence of defects in the dielectric layer may have a decisive influence on the properties of the cathodes. In this way we can obtain different results even when using the same materials for making the cathodes. The present paper deals with the influence of the thickness of the dielectric on the electrical properties of cathodes. Two different dielectrics were used, produced in different ways: a dielectric dependent on the metal electrode (oxidizing cathodes) and independent (evaporated cathodes).