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).
The form of electrodes and its influence on puncture voltages and dielectric strength in insulating materials.
| Identifier | ExFiles\Box 24\2\ Scan089 | |
| Date | 25th January 1921 guessed | |
| - ,2 - The form of the electrodes is of great influence. Text books often give very varied puncture values of the same materials, which is due to the fact that the test conditions, particularly in respect to the form of the electrodes, are not uniform. It depends upon whether points, balls or flat plates, are used as the electric force is proportional to the electric density. With sharp points this reaches a maximum and with spheres it is inversely proportional to the radius. It is of great importance to produce as homogeneous a field as possible, and on that account flat plate electrodes should be pressed as tight as possible against the dielectric so that any irregularities may become as small as possible in comparison to the resistance of the insulators. In spite of applying this pressure, disturbing effects are often met with as the lines of force have a tendency to bulge outwards. It is also impossible to avoid the presence of air between the electrodes and the insulating material, particularly at the edges of the electrodes, which are usually rounded off. If this is the case, effects are produced which will be explained later in the discussion of specific inductive capacity, electric discharges are produced in the air that is occluded and thus damage the dielectric and facilitate the puncture of the material. With direct-current higher puncture voltages are obtained, as in this case the effective and maximum voltages are the same. If a point is used as positive electrodes, and a plate as negative, then higher values are always obtained with alternating currents than with direct current. Large discs usually give lower puncture voltages as the weak spots in the dielectric are more certain to be discovered. It is further known that high stresses of short duration are less harmful to the material than low stresses of long duration. Investigations concerning the relationship between puncture voltages and dielectric strength have hitherto failed to disclose any certain interdependence between these quantities. Certain experiments assume that with certain limits, there is a linear relation between puncture voltage and thickness of material. On the other hand, Baur attempts to show by extensive tests that the highest value of the alternating current voltage is equal to the cube root of the square of the thickness of the material multiplied by the material constant - E = c.d (E = c.d (?) ). Nevertheless this formula cannot be taken as a law of nature. Kinsbrunner gives a formula differing from the above as follows - E = c.d ½. Variations of frequency have no effect on the punature voltage for low frequencies between 20 and 80 cycles a second. Heat is developed when insulating materials are subjected to high potential gradients. The heating due to the Joule effect increases rapidly as the displacement current | ||