An Improved Method of Making Permanent Electrets and Factors Which Affect Their Behavior

A method is described for making strong, smooth, and uniform electrets. The study concerns both the mixing of the carnauba wax and rosin of which the electret is made, and the manner of cooling in the electric field. The electret was cooled in a heavy iron cylinder placed in an oil bath whose rate of cooling was controlled automatically. Electrets were made for which the time of cooling ranged from 30 minutes to 10 days. Observations made on some 35 electrets seem to justify the following conclusions: (1) Electret behavior depends markedly upon details of preparation of the carnauba wax-rosin mixture. The time required for reversal of the electret's charge is increased by mechanical mixing of the components, by heating to a high temperature for a short time, or by heating to a somewhat lower temperature for a longer time. (2) Extending the cooling time in the electric field beyond a day produces little effect. Below this there exists a range of cooling times vital to the behavior of the electret. Those cooled in the open air in the customary manner reverse sign in a shorter time than do those cooled within the cylinder. X-ray photographs show a structural difference between these. (3) Alignment of crystals, as judged from x-ray studies, has little to do with the final charge density attained by the electret. Many samples which are electrets show no alignment. (4) The behavior of the carnauba wax-rosin electret is that of carnauba wax physically modified by the presence of the rosin.