MULTIPLEX TELEPHONY AND TELEGRAPHY — SQUIEE. 139 



receiving end of the cable line whether or not the high-frequency cur- 

 rent is present on the line, whereas tlds note, winch has to be searched 

 for in tuning and which was entirely tuned out when speech was best, 

 gave a very convenient method of testing for the presence of high- 

 frequency current. 



Having determined the general nature of this disturbance and its 

 comparative unimportance, no further investigation of it was consid- 

 ered necessary at that time. 



The next fundamental point to determine was whether or not at 

 these frequencies a telephone can receive enough energy to make it 

 operative for producing sound waves in air. 



Since the self-induction of a standard telephone receiver is high, 

 energy at these frequencies is effectively barred from it. In the wire- 

 less telegraph art, where the frequencies involved are from one hun- 

 dred thousand to several million per second, this problem has been 

 uniformly solved by the introduction of some form of detector for 

 electromagnetic waves, whose function is to transform the energy of 

 the high-frequency oscillations into other forms suitable to a type of 

 instrument such as a telephone receiver. 



The next step, therefore, consisted in introducing various forms of 

 detectors, such as are now used in wireless telegraphy, between the 

 telephone receiver itself and the energizing circuit. Since the fre- 

 quencies being here considered are entirely above audition it was 

 necessary, in order to produce a physiological effect, to introduce 

 another element in this transformation, viz, some method of modify- 

 ing the continuous train of sustained oscillations from the generator 

 into groups or trains, the period of which falls within the limits of 

 audition. This was accomplished by employing the regular forms of 

 automatic interrupters, such as are now used in wireless telegraphy, 

 with the expected result that with these two additional and essential 

 pieces of apparatus operatively connected between the telephone 

 receiver and the generator, the energy of the generator was delivered 

 to the ear in a form well suited for physiological effects. Since it is 

 well known that the human ear is most sensitive at a period of about 

 500 cycles per second, or 1,000 alternations, interrupters giving tins 

 frequency were employed. 



The presence of the detectors in this chain of transformations is 

 necessitated by the use of the telephone receiver as a translating 

 device. 



Although some of the detectors for electric waves are very sensitive 

 to electrical energy they are here employed not because they are more 

 sensitive to electrical energy than is the telephone receiver itself, 

 which is not the case, but because the telephone receiver is not adapted, 

 for the reasons stated above, to translate electrical energy of these fre- 

 quencies into movements of its diaphragm. 



