ABSTRACTS OF TECHNICAL ARTICLES 187 



Minimum Noise Levels Obtained on Short-Wave Radio Receiving 

 Systems.^ Karl G. Jansky. The theoretical minimum noise level 

 of receivers in the absence of any interference, the source of which is 

 external to the receiver, is discussed and compared with the limit 

 actually measured on various antennas over a limited frequency 

 range in the short-wave spectrum. It is pointed out that, on the 

 shorter wave lengths and in the absence of man-made interference, the 

 usable signal strength is generally limited by noise of interstellar origin. 

 The powers obtained from this noise with the various antennas and 

 for different times of the day are given. 



Recently, man-made interference, of which that caused by diathermy 

 machines constitutes the greatest part, has become so extensive that 

 it is now the limiting noise during most of the daylight hours. Data 

 are given on the intensity and extent of this form of interference. 



Superstructures in Alloy Systems.^ Foster C. Nix. A review of 

 the literature treating superstructures in alloys. The presence of a 

 superstructure produces new X-ray diffraction lines, — ^commonly 

 called superstructure lines. The elements of superstructure theories 

 are presented including the Bragg-Williams and Bethe-Peierls treat- 

 ments. The author discusses the effect of a superstructure or an 

 ordered phase on the thermal, mechanical, electrical and magnetic 

 properties of alloys. A comparison is made between the theoretical 

 predictions and the experimental results for both the specific heat and 

 the energy of transformation. A CusAu alloy becomes disordered 

 more rapidly near the critical temperature of order, Tc, than the 

 theories predict. A large anomalous specific heat is observed above 

 Tc, — as predicted by the Bethe-Peierls theory. 



Moment Recurrence Relations for Binomial, Poisson and Hyper- 

 geometric Frequency Distributions}'^ John Riordan. This paper 

 gives a uniform development of recurrence relations for moments about 

 the origin and mean of binomial, Poisson, and hypergeometric fre- 

 quency distributions. Uniformity is obtained through the use of the 

 moment arrays of H. E. Soper. Both types of moments are expressed 

 in terms of coefificients which are alike for the three distributions; for 

 the moments about the origin these coefificients are the Stirling num- 

 bers of the second kind. Moment recurrence relations follow from 

 recurrence relations for these coefificients. The recurrences for the 

 hypergeometric moments appear to be new. For working purposes, 



^Proc. I. R.E., December 1937. 



8 Jour. Applied Physics, December 1937. 



1" Annals of Math. Statistics, June, 1937. 



