THE HUMMING TELEPHONE, 



A Contribution to the Theoretical and Practical Analysis 



OF its Behavior. 



By a. E. KENNELLY and WALTER L. UPSON. 

 (Received July 20, 1908.) 



The following paper describes the salient features of an experi- 

 mental research on the humming telephone, conducted in the Gradu- 

 ate School of Applied Science of Harvard^ University during the 

 year 1907-08, and discusses an elementary mathematical riieory 

 which the observations appear to indicate and support. 



Definition. — A " humming telephone " is a connection of : 



1. A telephone receiver, or ordinary hand 'phone. 



2. A telephone transmitter, or ordinary carbon microphone. 



3. A source of electric power, such as a voltaic battery and tele- 

 phone induction coil, with the receiver in such electric and acoustic 

 relation to the transmitter, that it is able to emit a sustained note 

 or hum. This auto-excited hum may be so loud as to be heard in 

 a distant room through several partitions. 



Historical Outline. — The fact that a telephone receiver held, 

 either in contact with, or close to, the face of its transmitter may 

 cause the production of a hum or singing tone, appears to have been 

 first observed by Mr. A. S. Hibbard.- This experimental fact is 

 now well known to telephonists. In many cases, it is only necessary 

 to lift a subscriber's telephone from its hook, and hold it face to 

 face with its transmitter, in order to produce a loud hum. 



The only published investigation of the humming telephone that 

 the authors have succeeded in finding is an important paper by Mr. 



^ " Investigation of the Phenomena of ' The Humming Telephone,' " by 

 Walter L. Upson, a thesis towards the degree of master of science in elec- 

 trical engineering, Harvard University, 1908. 



" September, 1890. See Gill's paper hereafter referred to. 



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