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).
Technical page discussing the properties and challenges of insulating materials, referencing experiments by Tesla and Fessenden.
| Identifier | ExFiles\Box 24\2\ Scan091 | |
| Date | 25th January 1921 guessed | |
| - 4 - same potential as before. It might have been thought that the insulation between the poles would have been improved by the far higher dielectric strength of vulcanite as compared to air. Tesla concluded that vulcanite is a better conductor than air. This conclusion is not correct. The fact is that as vulcanite has a dielectric constant three times greater than air the potential gradient is much smaller in it. In consequence the shortened air-path is stressed more severely than before and sparks pass. This phenomenon was confirmed by Fessenden in 1908, using glass instead of vulcanite. The speed of electric effects in a dielectric is inversely proportional to the square of its dielectric constant. I wish to add that according to Maxwell a relation exists between the specific inductive capacity and the refractive index of insulating material and that the dielectric constant of a material is equal to the square of its refractive index. The experiments of Tesla and Fessenden show the great importance of attaining perfect homogenity of insulating material in their manufacture. Structure in layers and porous formations that might contain air or water must be avoided with the greatest care. Inequalities in mechanical strength are equally to be avoided. The formation of layers and pores interferes with the homogenity of the electric field and hinders the equal distribution of the electric forces. The fall of potential becomes concentrated upon the pores which begin to act like point-dischargers. The unreliability of many insulating materials and their early breakdown is often due only to faulty structure and unequal hardness of the material. If an electric insulator is stressed by applying a high alternating voltage, energy losses, that are small in good insulators, take place therein. These losses are generally of no importance in ordinary heavy current installations. In solid bodies they are proportional to the square of the voltage and to the capacity and frequency, and are due to the work required to reverse the polarisation of the dielectric, that is to say, with the exception of heat caused by radiation, they are due to molecular friction analogous to the magnetic phenomena associated with the reversal of magnetic fields. On this account the phenomena is known as dielectric-hysteresis in alternating electro-static fields. High frequency currents therefore tire the material more rapidly. The losses in liquid insulators, for instance transformer oil and paraffin, are however almost completely independent of the frequency which indicates losses due to electrolytic conduction. This supposition is strengthened by the fact that when a material passes from the liquid to the solid state, the influence of the frequency again appears. | ||