A. F. WALTER, Breakdown of Solid Dielectrics (from the series “Problems of the Newest Physics”), GTTI, 1933.

Until now there has been no even moderately satisfactory point of view on the mechanism of electrical breakdown. Despite the abundance of theoretical works on this question, among them one cannot point to a single one that would be free of contradictions. If the nature of the so-called thermal breakdown may already be regarded as sufficiently clarified, then in the field of the so-called electrical breakdown there reigns complete confusion of views. This seems all the more strange, since the modern theory of the solid state, already sufficiently developed, can serve as a fruitful foundation for constructing a theory of breakdown as a process of the transformation of a dielectric into a conductor under the action of a field.

Walter’s booklet is a concise survey written from the point of view of elucidating the mechanism of breakdown. In its critical part this survey is successful: it convincingly shows all the imperfections of the proposed theoretical views on breakdown. After a detailed analysis of the possible mechanisms and their elimination one after another, the phenomena of breakdown turn out to be deprived in general of any theoretical interpretation whatsoever. The author confines himself to vague hopes for quantum theory.

From this quantum point of view, breakdown should evidently be regarded as the transfer of electrons by the forces of the electric field into the conduction band. In the presence of a field the energy bands of a dielectric may be represented by inclined strips, and this is equivalent to the representation of a potential barrier that electrons must overcome in order to enter the conduction band. Here there arises a very essential and still unclear question: is breakdown caused by electrons of the metal (the electrode), or by electrons of the dielectric locked in the lower energy levels? One must wait until this question is resolved experimentally. (Here experiments with electrodeless breakdown would play the decisive role.)

However, the solution of the theoretical problem would be approximately the same whether we considered the seepage of an electron into the conduction band from the metal or from the lower levels of the dielectric itself. The problem would probably not present great difficulty if it concerned the oscillatory motion, that under the action of an electric field the energy bands of a dielectric themselves, one must think, are deformed in the same way as the energy levels of an individual atom placed in an electric field are deformed.

For the construction of a modern theory of breakdown, the results of recent experiments by Walter and Inge are very important; unfortunately, Walter does not mention them in the booklet. Walter and Inge studied the influence of illumination on breakdown. Contrary to expectations, light that increases the conductivity of a dielectric had no effect at all on the magnitude of the breakdown voltage.

Theoretical considerations on breakdown in the “quantum spirit,” although they are already in the air, have not yet been clearly formulated anywhere and therefore did not enter Walter’s booklet.

Walter’s booklet is a very clearly and concisely written critical article, which in no way can be regarded as an experimental survey. For that purpose it is better to turn to his Physics of Dielectrics, published in 1933. The corresponding chapters in Physics of Dielectrics are presented more fully and thoroughly than in this booklet. They also present and compare, less one-sidedly, the various theoretical views on the mechanism of breakdown. In comparison with Physics of Dielectrics, the booklet under review contains no new material, except perhaps the very interesting last paragraph, devoted to questions of aging. Here the question of the dependence of the breakdown voltage on the duration of the field action in the electrical breakdown of inhomogeneous (multiphase) dielectrics is considered.

If the mechanism of breakdown of homogeneous dielectrics in a homogeneous field is not clear to us, then one can imagine how much more complicated the question becomes when we turn to technical dielectrics and inhomogeneous fields. In the case of technical dielectrics the question is complicated also by the fact that, for them, the features of thermal and electrical breakdown become confused.

A rather large amount of space is devoted to technical dielectrics and inhomogeneous fields (as well as incomplete breakdown). The entire second half of the booklet is devoted to these questions.

The first half (homogeneous dielectrics in homogeneous fields) contains a very brief survey of the general regularities in breakdown (duration of exposure and temperature), an exposition of the basic ideas of the thermal and electrical theories of breakdown (less detailed than in Physics of Dielectrics) and their criticism (more detailed than in Physics of Dielectrics).

On the whole, Walter’s booklet is very interesting. It is interesting not so much for the novelty of its material as for its acuteness. After reading this booklet, the theoretical “breakthrough” in this field becomes quite evident.

F. F. Volkenstein