Kinds Of Insulators Research Articles

Electrical Breakdown Over Insulators in High Vacuum

The breakdown voltage of vacuum gaps depends on the electrode material, but when the gap is bridged by an insulator it is independent of the electrode material and varies with the kind of insulator. Some indication is found that the breakdown voltage increases with increasing surface resistivity of the insulator, but no apparent correlation is found between breakdown voltage and dielectric constant, density, or vapor pressure. It is found that a roughening of the intervening surface of the insulator in a region adjacent to the cathode increases the breakdown voltage. For electrode separations of 1 mm or greater the breakdown voltage does not increase linearly with the length of the insulator. Experiments were made in which one of the electrodes was separated from the insulator. It seems that the critical gradient at breakdown in the vacuum space between the cathode and the insulator, calculated for the case of plane electrodes from the ratio of dielectric constants, is not as large as would be required in gaps without insulators. These gradients are almost the same for copper as for stainless steel electrodes. The breakdown voltage over an insulator is raised when the edge of the insulator close to either electrode is rounded. When a layer of glass, thin compared with the separation of electrodes, is fused to the cathode, breakdowns occur at lower voltages than for an identical vacuum gap.

The Economical Thickness of Insulation in Refrigerator Cars

Abstract The authors call attention to the increasing importance of the economic design of refrigerator cars and discuss the factors which must be taken into account in finding the proper thickness of insulation. The outstanding feature of the paper is the use of alignment charts to determine readily answers to problems involving many variables. By aid of these, or similar charts, one may determine not only the economic thickness of insulation but also the saving which would result from the adoption of the correct thickness compared with any other thickness; furthermore, the economic advantage of one kind of insulation over another may be found. The paper gives a brief discussion on the economy of using regranulated cork or air spaces in some constructions. It outlines, by aid of average shade-temperature maps, the means of approximating external surface temperatures and notes the results of tests on roofs, sides, and floors as affected by direct sunlight and color of paint. Reference is also made to the analysis by the authors of the cost of transporting extra weight of insulation, or extra weight of cars as a whole, and the final average figure is found to be about 0.10 cent per ton-mile for transcontinental routes. All of the above and other factors form a part of the data for a complete solution for economic thickness, whether it is solved by a formula or by charts.

Discussion on “failure of disk insulators on high-tension transmission”

R. J. C. Wood: There is a very interesting thing in Table 3. In the early years we find that the No. 1 disk is the one which is failing to the greater extent, in 1913, 1914, 1915, 1916; and then in 1917 No. 7, which is the one nearest the point of support, usurps first place and keeps approximately that position, so that on the whole in these latter years it is the one nearest the point of support which shows the greatest number of failures. Now we all know that in the years 1913, 1914, the kind of insulator we were getting was very different from the insulators we get today; and that raised the question in my mind as to whether those early failures may not have been due to one cause — perhaps soft, porous porcelain, which was to be had in plenty; and the failures of the later years, when it is to be presumed that most of that poor stuff had been weeded out, have been caused by prolonged mechanical vibration.