126 BELL SYSTEM TECHNICAL JOURNAL 



crystal standard (likewise about 5,000 cycles) in a device which produces a 

 rotating magnetic field at the difference frequency. An armature which 

 follows this field controls the tuning condenser of the original electric oscil- 

 lator, coming to rest when exact synchronism is attained. The small 

 phase vibrations accompanying modulation are not followed because of the 

 inertia of the system. 



The stability thus obtained for the mean frequency is identically that of 

 a crystal oscillator. Since the actual control is mechanical, no sustaining 

 voltage is required, so that failures in the control system do not result in 

 sudden departures in frequency. Mechanical control, moreover, com- 

 pletely relieves the modulating elements of any connection with the stabil- 

 ization of the mean frequency, so that the modulation range is not re- 

 stricted. This and other refinements in design permit frequency excur- 

 sions of hundreds of kilocycles with extremely low distortion. 



UUra-Short-Wave Transmission Over a 39-Mile "Optical" Path.^ C. R. 

 Englund, a. B. Crawford, and W. W. Mumford. Continuous records 

 of ultra-short-wave transmission on wave-lengths of 2 and 4 meters, over a 

 good "optical" path, have shown variations in the received signal strength. 

 These variations can be explained as being due to wave interference; an 

 interference which varies with the changes in the composition of the tropo- 

 sphere. 



Some of the variations are due to changes in the dielectric-constant gradi- 

 ent of the atmosphere near the earth. Other variations are explicable in 

 terms of reflections from the discontinuities at the boundaries of different 

 air masses. The diurnal and annual meteorological factors which affect 

 the transmission are discussed. 



A Decade of Progress in the Use oj Electronic Tubes. Part I — In the Field 

 of Communication} S. B. Ingram. The dependency of the art of com- 

 munication on the science of electronics is so great as to make a review of 

 progress in electronics almost of necessity a review of the field of communi- 

 cations itself. While it is true that the early forms of telephone and radio 

 communication advanced to a degree without the use of electronic devices 

 as we know them today, the recognition of the vacuum tube as an amplifier 

 and generator of high-frequency alternating currents in the years just pre- 

 ceding the first World War marked a turning point in the development 

 of the communication art. From that day to this the progress of elec- 

 tronics and communications has gone hand in hand. The need of the 

 communications engineer for new electronic tools has kept him continually 



' Proc. I.R.E., August 1940. 



* Electrical Engineering, Transactions section, December 1940. 



