MULTIPLEX TELEPHONY AND TELEGRAPHY SQUIEE, 



145 



phone F' the detector D is operativery connected; E is the earth 

 connection. 



The local battery telephone sets are connected across the two line 

 wires in the usual manner. In both sets 1 is the microphone trans- 

 mitter; 2 is the local battery; 3 is the induction coil; 4 is the ringing 

 s} r stem, including the bell and hand generator; 5 is the switch hook; 

 6 is the telephone receiver. 



It was found that cross-talk was heard in the detector circuit from 

 the battel y tiansmitter at the transmitting end when the detector 

 circuit alone was connected directly to earth from the line without 

 any tuning coil or condenser. If, however, the tuning condenser was 

 inserted, this cross-talk entirely disappeared, even though the tuning 

 coil was not inserted. This is because the impedance of the small 

 tuning condenser is large for telephonic frequencies, while the tuning 

 coil impedance admits these telephonic frequencies. Both elements 

 of tuning are required for selective absorption of energ} r , so that the 

 high-frequency circuit is available as an additional telephonic circuit. 

 With this arrange- 

 ment talking in the 

 transmitter of the 

 high-frequency side 

 of the system was 

 heard only in the 

 detector and there 

 was no cross-talk 

 from the ordinary 

 local battery cir- 

 cuit. Similarly, 

 there was no effect of the high-frequency transmission on the local 

 battery transmission, and the two telephonic messages were com- 

 pletely separated. Both circuits were entirely free from earth dis- 

 turbances. 



The volume of speech at the receiving end of the cable is greatly 

 increased by simply inserting the transmitter in the dynamo circuit 

 and operating this circuit at or near resonance. In addition, the 

 coupling at both transmitting and receiving stations should be so 

 designed as to permit adjustment for optimum. 



The frequency used in this experiment was about 100,000 cycles 

 per second. The talk on the regular battery circuit was of the usual 

 high standard both ways, so that the only reason at this point why 

 complete duplex-diplex telephony was not obtained was the fact that 

 there was no high-frequency dynamo available at the laboratory. 

 There is, however, available at this laboratory one of the latest forms 

 of the high-frequency arc, and accordingly this was arranged with 

 suitable electromagnetic constants to give a period of about 71,000 

 38734°— sm 1911 10 



Fig. 4. 



