Bell's Telephonic Researches 



ent pitch are caused simultaneously to open and 

 close the primary circuit a double tone proceeds 

 from the spark. 



A curious discovery, which may be of interest 

 to you,, has been made by Professor Blake. He 

 constructed a telephone in which a rod of soft 

 iron, about six feet in length, was used instead 

 of a permanent magnet. A friend sang a con- 

 tinuous musical tone into the mouthpiece of a 

 telephone, like that shown in Fig. 12, which was 

 connected with the soft iron instrument alluded 

 to above. It was found that the loudness of the 

 sound produced in this telephone varied with the 

 direction in which the iron rod was held, and 

 that the maximum effect was produced when the 

 rod was in the position of the dipping needle. 

 This curious discovery of Professor Blake has 

 been verified by myself. 



When a telephone is placed in circuit with a 

 telegraph line the telephone is found seemingly to 

 emit sounds on its own account. The most 

 extraordinary noises are often produced, the 

 causes of which are at present very obscure. 

 One class of sounds is produced by the inductive 

 influence of neighbouring wires and by leakage 

 from them, the signals of the Morse alphabet 

 passing over neighbouring wires being audible in 

 the telephone, and another class can be traced 

 to earth currents upon the wire, a curious modifi- 

 cation of this sound revealing the presence of 

 detective joints in the wire. 



Professor Blake informs me that he has been 

 81 



