MULTIPLEX TELEPHONY AND TELEGRAPHY SQU1EK. 141 



D. C, this wire was connected directly to earth through a "perikon" 

 crystal detector, such as is well known in wireless telegraphy, and a 

 high resistance telephone receiver of about 8,000 ohms was shunted 

 around the crystal. In this preliminary experirrient no attempt was 

 made at tuning, either at the transmitting end or at the receiving 

 end of the line. 



In the primary circuit of the generator, arrangements were made 

 by which either an interrupter and telegraph key or a telephone 

 transmitter could be inserted by throwing a switch. 



In the line circuit a hot wire milliam meter was inserted in a con- 

 venient position so that the effect of the operation of either the 

 telegraph key or of the human voice upon the transmitter could be 

 observed by watching the fluctuations of the needle of the milliam- 

 meter. 



A loose coupling was employed between the two circuits at the 

 transmitting end, and the line circuit adjusted by varying the coup- 

 ling until the current in the line was 20 to 30 milliamperes. With 

 this arrangement (1) telegraphic signals were sent and easily received, 

 and (2) speech was transmitted and received successfully over this 

 single wire with ground return. 



The ammeter showed marked fluctuations from the human voice 

 and enabled the operator at the transmitting station to be certain 

 that modified electric waves were being transmitted over the line. 



The actual ohmic resistance of the line apparently played an 

 unimportant part for telegraphy at 100,000 cycles, since with one 

 of the wires of the pair and a ground return, the effect of doubling 

 the conductivity of the wire by joining both wires in parallel, although 

 this arrangement increased the capacity of the wires, could not be 

 detected with certainty by an operator listening to the signals and 

 unaware of which arrangement was being used. 



Inserting in the line wire a noninductive carbon rod resistance 

 of 750 ohms, which is practically the resistance of the line itself, 

 could not be detected by any change in the intensity of the received 

 signals. 



The next experiment was to determine what effect, if any, such 

 sustained electrical oscillations would have upon the minute tele- 

 phonic currents employed in battery telephony. 



DUPLEX TELEPHONY, USING ONE GROUNDED CIRCUIT. 



To determine the fact that electric waves of ultra sound frequency 

 produce no perceptible effect when superimposed on the same circuit 

 over which telephonic conversation is being transmitted, the next 

 step was to use such a train of sustained oscillations as the vehicle 

 for transmitting additional speech over the same circuit. For this 



