266 BELL SYSTEM TECHNICAL JOURNAL 



originally with regard to duplex operation, the possible advantage of 

 applying duplex apparatus to it would obviously be greater than it 

 is for any of the existing loaded cables. 



There are many ways in which the design of a cable might be 

 modified to make duplex operation more advantageous than it is on 

 the present cables, and the problem is much too complex to permit 

 very detailed discussion here. Improvements in constancy of in- 

 ductance with variation in current and in reduction of alternating- 

 current resistance factors would be of obvious advantage. A tapered 

 cable with high inductance in the middle and low inductance at the 

 ends would also have advantages in this connection and to a limited 

 extent tapering has already been applied to the Pacific Cable Board's 

 loaded cables by arranging the component parts of these cables during 

 manufacture with regard to their inductance. 



One of the most attractive methods of duplexing, which would also 

 provide great flexibility of operation, is to use carrier current operation 

 in one direction and ordinary telegraph operation in the opposite 

 direction. Non-loaded cables are ordinarily duplexed by balancing 

 the cable at each end with an artificial line which permits separation 

 of the weak incoming signals from the strong outgoing ones and the 

 limit of speed is usually set by the accuracy with which the cable may 

 be balanced by the artificial line. By using carrier current operation 

 in one direction and ordinary telegraphic operation in the opposite 

 direction the incoming and outgoing signals may be separated by 

 the combined use of artificial lines and frequency filters, in the manner 

 long since employed on the Key West-Havana cables for carrier 

 currents above the voice-frequency range. To design a cable for 

 carrier current operation would, of course, require consideration of its 

 behavior at much higher frequencies than those employed on the 

 existing long loaded cables and would probably call for very high 

 resistivity loading material applied in a very thin layer. 



The recent spectacular development of radio both for telephony 

 and telegraphy has raised in the minds of all the question as to whether 

 there is any future left for ocean cable telegraphy. Opinions on this 

 question will doubtless differ. My own opinion is that for short 

 distances across the sea, where the demand for communication is 

 considerable, cables always have offered more economical and satis- 

 factory communication than radio and will probably continue to do so. 

 For long over-sea distances I believe the cables would have faced a 

 serious situation in competition from radio had not permalloy loading 

 been brought forth. Now permalloy loading has so reduced that 

 part of the total cost per word for which the cable itself is responsible 



