528 BELL SYSTEM TECHNICAL JOURNAL 



Formation of Photographic Images on Cathodes of Alkali Metal 

 Photoelectric Cells.^ A. R. Olpix and G. R. STiLwr-:LL. A method 

 of forming both negative and positive photographic images on the 

 cathodes of potassium and sodium photoelectric cells in vacuum is 

 described. These images are sharp and clear in every detail and can 

 be permanently "fixed" by proper treatment. Among the materials 

 which have been successfully used in treating the exposed surfaces to 

 bring out these images are sulphur vapor, air, oxygen and hydrogen 

 in the ratio of 9 to 1, hydrofluoric acid and bromine. During the 

 time the image is forming, the photoelectric sensitivity of the illumi- 

 nated portions decreases approximately 30 per cent. After the image 

 is fixed as a permanent record there is little difference between the 

 sensitivity of the cathode area bearing the image and neighboring 

 areas. Photographs of photoelectric cells are shown in which such 

 photographic images are plainly visible. 



Ausgleichsstrome bet parallelen Einzelleitimgen, von denen die eine in 

 der Erde liegt und unendlich lang ist}^ John Riordax. This paper 

 gives the formula for the electric force in a homogeneous semi-infinite 

 fiat earth due to unit step current (zero for time less than zero, unity 

 for time greater than zero) in an infinite wire above the earth. The 

 corresponding formula for the electric force in the air, due to F. H. 

 Murray, has been published in the Bell System Technical Journal for 

 October, 1930, equation (4) of L. C. Peterson's paper; the two formulas 

 agree at the surface of the earth. The present formula is given in 

 finite form in terms of the exponential function and the error function 

 complement. 



A Modern Laboratory for the Study of Sound Picture Problems}^ 

 T. E. Shea. Recently there has been provided among the research 

 facilities of Bell Telephone Laboratories, Inc., a separate building 

 which is intended solely for sound picture research and development 

 work. The prime objects of the laboratory are to find out the best 

 methods and technic for employing sound picture recording and 

 reproducing apparatus now in use, and of making improvements in 

 recording and reproduction. The building contains a recording studio, 

 film processing plant, and review room, together with testing labora- 

 tories. 



3 Jour. Opt. Soc. Amer., March, 1931. 

 ^°E. N. T., Band 8, Heft 3, March, 1931. 

 11 Jour. S. M. P. E., March, 1931. 



