MAGNETO-ELASTIC SOURCE OF NOISE 339 



as bcinj; novel and curious. So direct a means of converlinj; me- 

 chanical into electrical energy should find some useful applications. 

 ;\s the sensitive element in a telej^hone transmitter it should l)e added 

 to the considerable list of other devices which ha\e been used for this 

 and allied purposes. The following arrangement was constructed. 

 A fine iron wire was laced back and forth between pegs located along 

 the opposite edges of a five-inch sciuare opening in a wooden frame so 

 as to screen the aperture with one hundred spans of the wire all in 

 series, evenly spaced, and parallel. A sheet of paper then was ce- 

 mented to the screen and the two ends of the wire were connected to 

 the amplifier input. This device performed as a crude telephone 

 transmitter. It was recognized, of course, that with this simple 

 arrangement the iron wire would undergo two complete cycles of stress 

 for each complete cycle of the air pressure upon the diaphragm, and 

 that mechanical bias or some equally effective means would have been 

 necessary to eliminate this distortion. Where the vibrating element 

 itself was of magnetic material, there was the possibility of the sound 

 source constituting its own transmitter. For example, when the 

 amplifier input w^as connected between the bridge and key-head of a 

 steel-stringed guitar the music of this instrument was reproduced 

 quite faithfully from the speaker. This was suggestive of the pos- 

 sible use of the effect in studying the vibrations and strains occurring 

 in steel structures such as bridges. 



References 



1. "The Bell Telephone," American Bell Telephone Company, Boston, 1908, p. 54. 



The following quotation from Bell's testimony was called to the writers* 

 attention by Mr. R. I. Caughey. Yes. I tried such an experiment, I think in 

 the year 1879. A continuous current from a voltaic battery was passed through a 

 stretched wire — I think a thin steel wire — which was placed in the same circuit 

 with an ordinary commercial hand telephone. When the wire was plucked by 

 hand, it vibrated, producing a musical tone. The hand telephone was in another 

 room, and I listened at the telephone while a young man was plucking the wire. 

 I heard a musical tone from the telephone at each pluck, and could recognize, also, 

 that the character or "timbre" of the sound produced by the vibrating wire was 

 reproduced by the telephone at my ear. 



2. Ewing, "Magnetic Induction in Iron and Other Metals," 3d ed., 1900, Chap. IX. 



3. Von Hippel and Stierstadt, Zeitschrift fur Physik, 69, 52 (1931). 



Von Hippel, Stierstadt, and von Auwers, Zeitschrift fur Physik, 72, 266 (1931). 



4. Von Auwers, Zeitschrift fur Physik, 78, 230 (1932). 



5. From the results at this preliminary stage the present writers were convinced that 



the electromotive forces appearing at the terminals of the vibrating wire 

 were determined Ijy the cycles of strain, and not by the cycles of displacement, 

 of the specimen. It seemed to them that this fact, and this fact alone, was 

 demonstrated by von Auwers' elaborate study of the frequency and phase 

 relationships between the mechanical vibration and the terminal electromotive 

 forces. 



6. Ewing, Chap. IX, P- 234. 



