ABSTRACTS OF TECHNICAL ARTICLES 251 



that will make them more easily available to the practical engineer, and to 

 till the gaps in these results by developing a series of curves from which the 

 field for any values of the earth's constants may be read, with all the ac- 

 curacy that could be expected in engineering practice. 



Electrical Breakdown of Anodically Oxidized Coatings on Aluminum: A 

 Means of Checking Thickness of Anodized Finishes? K. G. Compton and 

 A. Mendizza. The existing methods for determining the thickness of 

 anodically produced oxide coatings on aluminum are relatively few and are 

 almost entirely of a destructive nature. It is a fairly well established fact 

 that, within the thickness limits normally encountered in practice, the 

 voltage breakdown is a linear function of thickness of oxide film. The 

 authors have endeavored to utilize this fact in developing a test method 

 for determining the thickness of coatings produced under known and con- 

 trolled conditions with practically no injury to the finish. Data are given 

 which show the relationship between breakdown resistance, anodizing time, 

 thickness of coating, current density and sealing of anodically oxidized 

 polished commercially pure aluminum. Statistical data for the values ob- 

 tained are also given, indicating the good reproducibility of the breakdown 

 values. By calibrating a particular anodic process, satisfactory results 

 may be obtained in a relatively short time and often without destroying or 

 marring the article. Since the oxide coating is not entirely homogeneous 

 it is necessary to obtain a fairly large number of readings for every test 

 condition. The authors have found that approximately twenty-five read- 

 ings are usually sufficient and can be made in a relatively short time. Al- 

 though only one of the many anodizing possibilities has been investigated, 

 the applicability of this method of evaluating the thickness of oxide coatings 

 may be extended to all commercial treatments. 



Ultrasonic Absorption and Velocity Measurements in Numerous Liquids} 

 Gerald W. Willard. By means of ultrasonic light-diffraction phenomena 

 the velocity and absorption of sound in some forty transparent liquids were 

 measured in the frequency range of 6 to 30 Mc. Among the list of materials 

 studied are mixtures of liquids in varying proportions, several solutions of 

 solids in liquids, and a non-liquid jell. A novel-construction glass-to-metal- 

 to-quartz cell made possible the study of highly solvent liquids. Velocity 

 values w^ere obtained from measurements of the diffraction spectra spacing. 

 Absorption values were obtained by measurement of the sound radiator 

 voltages required to produce certain color transmission effects at measured 

 distances from the sound radiator. The use of a mercury arc light-source 



^A.S.T.M. Proc, Vol. 40, 1940. 



*Jour. Acous. Soc. Amer., January 1941. 



