From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Dielectric breakdown, potential gradients, and the influence of temperature and specific inductive capacity on insulating materials.
| Identifier | ExFiles\Box 24\2\ Scan090 | |
| Date | 25th January 1921 guessed | |
| - 3 - grows with the increasing potential gradient, the destruction of the dielectric results and the temperature increases as long as the stored heat is greater than the radiated heat. Dark discharges, spark and brush discharges, with hissing noise, creeping of the current, production of crackling sparks precede a puncture; also ready at 2000 volts a distinct smell of ozone becomes noticeable and nitric oxides are produced, that not only damage the insulating materials, but also attack metallic conductors, for instance, copper wires. The puncture is associated with chemical changes, such as carbonising, depositing of solids and liquids. The large quantity of water and resinous and other particles deposited during the moment of puncture, is very noticeable with some insulating materials. My experience indicates that the origin of this phonomenon lies in the genration of electro-osmotic forges, which might be expected to have an influence on the substance of materials exposed to electro-static strains. If different kinds of bodies that are generally insoluble are contained in a liquid in fine division or in susession these particles carrying charges begin to move under the influence of a potential gradient in the same way as the ions of salt. It should therefore be noted that these materials do not get deposited according to their chemical equivalents, but in most cases in much larger quantities. The higher the voltage and the shorter the distance between the electrodes, the quicker is the deposition of the colloid. If heat is developed in an insulating material through a high potential gradient, liquisifaction appears and the conditions for the above mentioned phenomena are produced. During the process the material with high specific inductive capacity charge themselves by contact with materials of lower positive specific inductive capacity, and are carried towards the cathode. As water has the very high dielectric constant of 80, the solution becomes weaker at the negative electrode, and water is deposited there. As the temperature and humidity of the dielectric increase in the manner explained above, a considerable decrease of its puncture strength results. We have extensive experimental results on this point, mostly in graphical form. The specific inductive capacity is without direct influence on the dielectric strength. Difference in the dielectric constants have only an effect if various insulating materials are connected in series. Tesla made an interesting test with reference to this. He placed two poles connected to a high potential difference at such a distance apart that sparks just failed to pass throughbthe air. He then connected plates of vulcanite as covers over the poles, the distance between them remaining the same, with the surprising result that an active discharge of sparks took place at the | ||