MULTIPLEX TELEPHONY AND TELEGRAPHY SQUIER. 151 



employ telegraphy as a quick and ready means of determining 

 resonance between the circuits in each particular case. 



When any particular arrangement was being employed the first 

 steps were invariably to send simple Morse signals over the circuit- 

 until the operator at the distant end of the line reported maximum 

 loudness in the receiving telephone, which indicated that the ter- 

 minal apparatus with the line circuit was properly tuned. This 

 being accomplished, it was necessary only to throw a switch to 

 substitute for the automatic interrupter and telegraph key the 

 telephone transmitter, and the experiments could then proceed on 

 telephony without any material change being made at the receiving 

 station. Telephony and telegraphy thus proceeded hand in hand 

 as a mere matter of convenience, and one of the practical advantages 

 in the use of electric waves for transmitting intelligence is that the 

 whole set-up of apparatus is practically the same for each and they 

 can be used interchangeably over the same circuit. 



Considering the Morse equipment, indicated in figure 8, the electro- 

 magnetic units involved are of the order of magnitude of microfarads 

 and henrys, and the period of the interrupted direct current for 

 Morse sending is not more than the equivalent of about 10 complete 

 cycles per second, whereas in the high-frequency side of the circuit 

 the electromagnetic units are of the order of magnitude of thousandths 

 of a microfarad and of thousandths of a henry and with frequencies 

 not less than 2,000 times greater than those involved in manual 

 Morse sending. Furthermore, the ohmic resistance of the line which 

 plays a prominent part in limiting the distance and speed of Morse 

 working, is comparatively unimportant in the case of electric waves 

 guided by wires. The operation of the line equipped as in figure 8 

 was perfectly satisfactory, there being no perceptible interference 

 between the two messages in either direction. 



Since the standard telegraph circuits of the world use a ground 

 return, this same equipment was arranged to operate on one of the 

 wires of the twisted-pair in the telephone cable as such a circuit 

 with earth connections at each end, and its operation was equally 

 successful. 



Since it is a well-known characteristic of high-frequency apparatus 

 used in tuned circuits that there shall be no iron involved in the 

 circuit, it is evident that in cases where such a high-frequency cur- 

 rent is to be superimposed upon a line comprising way stations, 

 where line relays are inserted directly in the circuit, it will be neces- 

 sary and sufficient to shunt such way stations by condensers of the 

 order of magnitude of thousandths of a microfarad. Such con- 

 densers offer a comparatively free path for the high-frequency electric 

 waves, but interpose a practical barrier to the Morse frequencies. 



