ABSTRACTS OF TECHNICAL ARTICLES 77 



their relation to the method of "paired echoes" is discussed. A restate- 

 ment is made of the condition which is known to be necessary in order 

 that a given steady-state characteristic may represent a passive or stable 

 active system (actual or ideal). A particular necessary condition is deduced 

 from this as an illustration. 



Paper Dielectrics Containing Chlorinated Impregnants.^ D. A. McLean, 

 L. Egerton, G. T. Kohman and M. Brotherton. Chlorinated aromatic 

 hydrocarbons are used extensively as impregnants for paper in electrical 

 apparatus such as capacitors and transformers. They possess high stability 

 toward heat and oxygen, nonflammability, and desirable electrical proper- 

 ties. However, the present study shows that under special conditions 

 a type of decomposition occurs which liberates products so active chemically 

 that, especially at elevated temperatures, under high potential gradients, 

 and in contact with metals, autocatalytic decomposition reactions are 

 initiated. 



When aluminum electrodes are used and d-c. fields are applied, the 

 deterioration manifests itself in a leakage current increasing with time and 

 in the formation of partially carbonized areas in the paper. Evidence is 

 presented that the decomposition starts by the splitting ofif of hydrogen 

 chloride from the chlorinated compounds. This attacks the electrodes to 

 form aluminum chloride, which decomposes the organic constituents of 

 the insulation. 



The deterioration is considered in relation to certain variations in com- 

 position; all components of the structure — the paper, the impregnant, and 

 the electrode materials — play important roles in the reactions. 



Methods for Measuring the Performance of Hearing Aids.^ Frank F. 

 RoMANOW. A hearing aid can be considered as a sound transmission 

 system which is interposed in the path between the source of sound and 

 the listener's ear. As such its performance can be judged by comparing 

 the sound that reaches the ear first through the air path and then through 

 the hearing aid. The experimental procedure to carry out this concept is, 

 however, time consuming. To obtain a simple laboratory technique for 

 comparative purposes, it is proposed, therefore, that the amplification of 

 a hearing aid be measured by placing its microphone in a known sound field 

 and observing the output of the receiver when terminated in an appropriate 

 impedance. For an air conduction receiver this impedance takes the form 

 of a closed cavity. For a bone conduction receiver the termination is an 

 artificial mastoid. Since in a portable hearing aid compactness is desired, 



^ Indus. & Engg. Cliem., January 1942. 

 ^Jour. Acous. Soc. Amer., January 1942. 



