ABSTRACTS OF TECHNICAL ARTICLES 311 



widths vary considerably with the treatment of the surfaces. It is 

 concluded that the values obtained from the electron measurements 

 apply to projecting surface metal only, and that the degree of misalign- 

 ment is much greater at the surface than deep down within the crystal. 

 Furthermore, even the x-rays [Mo Ka radiation — 0.71 A] are not 

 sufficiently penetrating to yield values certainly characteristic of these 

 metal crystals. 



Mutual Impedance of Grounded Wires Lying on the Surface of the 

 Earth when the Conductivity Varies Exponentially with Depth.^ Marion 

 C. Gray. This paper presents a formula for the mutual impedance of 

 any insulated wires of negligible diameter lying on the surface of the 

 earth and grounded at their end-points, on the assumption that the 

 conductivity of the earth varies exponentially with depth. Various 

 special cases are briefly discussed. 



Signals and Speech in Electrical Communication. "^ John Mills. 

 This book is written by a member of the technical staff of Bell Tele- 

 phone Laboratories who is well-known for his text on "Radio Com- 

 munication" (1917) and the more popular presentations of "Within 

 the Atom" (1921) and "Letters of a Radio-Engineer to His Son" 

 (1922). In this book he presents for the general reader a synthesis of 

 the electrical arts of communication in terms of their general funda- 

 mental principles. In separate chapters, which are discrete essays in 

 popular and semi-technical language, the fundamental principles of 

 dial operation, transmitters and receivers, loading coils, repeaters, 

 multi-channel or carrier systems, and transoceanic radio-telephony are 

 graphically expounded. The entertaining treatment of engineering 

 achievements in allied fields of the sound picture, broadcasting, tele- 

 vision, stereophonic reproduction and the teletypewriter, will intrigue 

 the layman and assist him in acquiring a general understanding of 

 these highly technical developments. 



Some Earth Potential Measurements Being Made in Connection with 

 the International Polar Year.^ G. C. Southworth. For several years 

 the Bell System has been studying the relation between radio trans- 

 mission and earth potential disturbances. A paper dealing with this 

 subject was published in 1931. Prompted by the needs of the Inter- 

 national Polar Year, together with the prospect that further work 

 would throw additional light on the nature of radio transmission, the 

 work was extended somewhat in 1932. 



^Physics, January, 1934. 



' Published by Harcourt Brace and Company, New York, N. V., 1934. 



» Proc. I. R. E., December, 1933. 



