Compressed magnesia exhibits a fairly high breakdown strength of 30o kv./cm. This high strength appears remarkable, in considering the large porosity factor of the wafers. The contradiction can be explained by the inorganic nature of the material, and its consequent resistance against overheating and ionization.
- INTRODUCTION.
Recently the authors have investigated the electrical properties of fibrous glass, 1 this new insulating material, the qualities of which furnish a striking proof for the superiority of inorganic insulators as compared with organic ones. Unfortunately there does not up to the present exist a plastic, homogeneous inorganic dielectric material, however desirable it would be both in combination with fiber glass and alone.
In this connection the authors have noted a paper of M. J. Bethenod, 2 describing the so-called Pyrotenax cable with compressed magnesia insulation, and mentioning that this dielectric, if subjected to deformation, behaves as a homogeneous, plastic material. Although the analogy to a plastic is evidently only external, it was thought worth while to further investigate this insulator, first of all its electric strength, the more so, as there are no quantitative data in this respect available in Bethenod's paper.
As the chief advantage of magnesia is its incombustibility, the cables just mentioned, like the heating units, known as Calrod, made with magnesia, are intended for high currents, and not for high voltages. Still, it might be useful to look upon this material as an insulator for reasonably high voltages, and investigate its behavior in this respect.