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The steady increase in the severity of operating conditions for capacitors and the need for more diverse characteristics has spurred the search for new and better capacitor dielectrics. The synthetic plastics industry, which is the source of a number of useful dielectric materials, has recently produced a new and promising dielectric in film form known as polyethylene terephthalate or ?Mylar.? This material is unusually strong, has a high softening point, and is available in very thin films which makes it especially suitable for capacitor insulation. The electrical characteristics of capacitors wound with Mylar film are likewise promising. As compared to mineral-oil-impregnated paper capacitors, unimpregnated Mylar capacitors exhibit higher dielectric strength, higher insulation resistance, and can be operated at higher ambient temperatures. Their loss characteristics are comparable with those of impregnated paper and the capacitance stability over the usual range of ambient temperatures approaches that of mica capacitors. Mylar is relatively nonhygroscopic and tests indicate that for moderate atmospheric conditions capacitors made from it do not require additional moisture protection. The film can be metallized readily by current techniques and, when used in metallized capacitors, appears to possess advantages over metallized paper in several respects.

Silver migration may be defined as a process by which silver, when in contact with insulating materials under electrical potential, is removed ionically from its initial location, and is redeposited as metal at some other location. This process requires adsorption of water on the insulation surface. Silver is unique in this respect in that it is easily oxidized and reduced and does not passivate. Other metals do not present a practical migration hazard. Presented herein are examples of actual experience wherein silver migration caused trouble, and an explanation of this phenomenon based upon chemical and physical considerations and related laboratory evidence. It is concluded that silver should be used with great caution under the conditions noted.

Penetrability studies have been made to measure the wetting of dielectrics, in an effort to secure information on the electrical characteristics of impregnated fibrous structures. The results of these studies are discussed in this article, which includes studies of the penetration of water, benzene, and several insulating materials.

In order to determine the relation between natural aging and oxygen absorption at 80° C., the effects upon oxygen absorption of a number of factors which are known to affect natural aging have been studied. An apparatus is described with which it is possible to follow the absorption of oxygen at constant pressure, temperature, etc., over long periods of time. It has been found that the absorption of oxygen is autocatalytic in nature and appears to consist of at least two separate reactions—a decomposition and an addition reaction. The rate of absorption of oxygen at 80° C. is dependent upon surface area when the area is small, but when it is increased to as large as 40 square centimeters per gram, small variations in surface area are without great effect, probably because under these conditions rates of diffusion are high enough to keep the rubber particles practically saturated with oxygen. As rubber absorbs oxygen it appears to undergo changes similar to those occurring during vulcanization with sulfur. Surface checking is regarded as a preferential surface oxidation. The mechanism of oxygen absorption appears to be different for raw rubber from that for cured rubber. Curing rubber increases both the rate and amount of oxygen absorption over that for raw rubber, and changes the character of the absorption curve. The rate of absorption increases with the degree of cure. The action of anti-agers is one of negative catalysis rather than one of preferential absorption. The rate of oxygen absorption may be greatly retarded by their presence and their effectiveness varies with their concentration and constitution. The type of absorption of ozonized oxygen differs markedly from that for pure oxygen. The former is taken up very much more rapidly at first, but the rate rapidly decreases and less oxygen is finally taken up than in the case of pure oxygen. The action of ozonized oxygen upon rubber appears to protect it from the action of pure oxygen. The reaction between rubber and ozonized oxygen is not autocatalytic.

Polyethylene terephthalate, or “Mylar,” is a new rival of paper for use as the dielectric in electric capacitors. It appears superior in regard to insulation resistance, temperature coefficient of capacitance, and operating temperature range.

The great interest recently shown in the use of sulfur hexafluoride as a high-dielectric-strength gas has been accompanied by serious consideration of the possibility of the production of toxic and corrosive products in the event of electric breakdown of the gas. In this paper we shall formulate a mechanism for the decomposition and report our findings regarding the products formed under various conditions including electric stress below the corona point.