ABSTRACTS OF TECHNICAL ARTICLES 399 



Filtered Thermal A^oise — Fluctuation of Energy as a Function of Interval 

 Length.^ S. O. Rice. Let a source of thermal noise be connected to the 

 input of a band-pass filter. Consider the energy which would be dissipated 

 during the interval /i to /i + T if the output current were to flow through a 

 resistance of one ohm. When T is held fixed and /i regarded as a random 

 variable, the resulting energies have a distribution whose average and stand- 

 ard deviation depend upon T. Here this dependence is studied. The 

 standard deviation of the difference of the energies of two contiguous inter- 

 vals, each of length T, is also obtained. 



Ultra-Short Electromagnetic Waves. IV — Guided Propagation.^ S. A. 

 ScHELKUNOFF. Doctor S. A. Schelkunoff presented the material contained 

 in this article as a lecture before the basic science group of the New York 

 Section. He treated the subject in a "non-mathematical" manner. Cer- 

 tainly the electrical engineer will welcome any concept which allows an 

 easier approach to the solution of certain problems involved in wave guides 

 than the more complete equivalent field-theory method. In this fourth 

 article in a series of six on ultra-short electromagnetic waves, Doctor Schel- 

 kunoff combines transmission-line theory with optical analogy and derives 

 useful relations for both wave guides and cavity resonators. The three 

 preceding articles appeared in the March, April and May issues of Electrical 

 Engineering. 



Variahle-Frequency Bridge-Type Frequency-Stabilized Oscillator s.'^ W. G. 

 Shepherd and R. O. Wise. Results are given of a theoretical and experi- 

 mental investigation into two types of bridge-stabilized oscillators incorpo- 

 rating a thermal device for amplitude control. One circuit employs only 

 resistances and capacitances in the frequency-determining network and 

 consequently is useful for low-frequency operation. The other circuit uses 

 an inductance-capacitance network which is well adapted to the higher- 

 frequency network. Conditions for optimum stability and the variation of 

 the stability with frequency determined experimentally are found to be in 

 general agreement with theoretical results. 



Beyond the Ultra-Short Waves.^ G. C. Southworth. This article reviews 

 briefly the work done many years ago by the pioneering physicists with the 

 so-called electric waves as well as the more recent efforts by engineers to put 

 these waves to practical use. It also describes some of the expedients and 

 changes of technic used to overcome difficulties as this work progressed to 



^ Jour. Acous. Soc. Amer., April 1943. 

 ^ Elec. Ens,g., June 1943. 

 7 Proc. I. R. E., June 1943. 

 « Proc. I. R. E., July 1943. 



