A Magneto-Elastic Source of Noise in Steel Telephone Wires 



By W. O. PENNELL and H. P. LAWTHER 



The appearance of an electromotive force at the terminals of a vibrating 

 rod or wire of magnetic material was investigated. It was concluded from 

 experiments somewhat more simple and direct than those employed by other 

 investigators that the effect was due to changes in the state of circular 

 magnetization accompanying the variations of stress. The results suggested 

 problems for more intensive investigation and applications of possible 

 practical value. 



nr^HERE probably are few persons who have not had the experience 

 ■^ of standing near some telephone or telegraph line out in the open 

 and hearing the singing of the wires resulting from the wind blowing 

 over them. Perhaps in childhood it was a source of wonderment why 

 these sounds could not be heard at the telephones, and later, upon 

 learning that telephone transmission was accomplished electrically 

 and that these vibrations were mechanical only, tolerant amusement 

 was felt at this earlier ingenuousness. Apparently it has remained 

 until a very recent date for the discovery unmistakably to be made 

 that it is possible under certain conditions for the mechanical vibration 

 of a telephone wire to generate electromotive forces of sufficient mag- 

 nitude to become objectionably audible in the telephone circuit. It 

 was in April, 1935 that Mr. G. G. Jones of the Long Lines Department 

 of the American Telephone and Telegraph Company mentioned to one 

 of the writers the experience his Company had had a short time before 

 in tracing down a supposed case of inductive noise to the action of 

 the wind upon a 1200-foot steel-wire river-crossing span near Topeka, 

 Kansas. This particular case had been cleared promptly by the 

 application of suitable vibration damping devices generally recom- 

 mended for situations where vibration might cause trouble. Special 

 investigations then were made of long steel-wire spans at several 

 locations in the Southwestern Bell Telephone Company territory, and 

 it was revealed that some slight noise derived from this source actually 

 was present in every case — and in one particular instance, where the 

 wind velocity and direction happened to become very favorable to the 

 production of wire vibration during the time of the inspection, the 

 noise arose to a serious magnitude. In none of these locations had 

 there been previous evidence that vibration was serious. That so 

 simple and direct a phenomenon had escaped identification at the 

 hands of telephone workers through the years of the art's existence 



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