77//. i.o.inr.ii sciiM.ikixi i ni.icK.irn (.inir. .w 



oondiiitors. Thi- o|KT;itinn spii-d of a non-loailctl inhlv is approxi- 

 mately inversi'ly proportional to the product of the total resistance 

 l)V the total capacitv-: that is, 



5= *-^ 

 CRP' 



where C is capacity anil R resistance per unit lenRlh, and / is the 

 length of the cable. The coerticient k is generally referred to as the 

 speed constant. It is. of course, not a constant since it depends on 

 such factors as terniinal interference and method of operation, hut 

 is a convenient basis for comparing the efticiencj' of operation of 

 cables of different electrical dimensions. .As the techni(|ue of oper- 

 ating cables has improved the accepted value of k has increased, its 

 value at any time being dependent on the factor then limiting the 

 maximum speed obtainable. This factt)r has at times been the 

 sensiti\eness of the receiving apparatus, at other times the distor- 

 tion of signals, anti in recent years interference. During a great part 

 of the history- of submarine cable telegraphy distortion was considered 

 the factor which limited the speed of operation of long cables and on 

 this account most of the previous discussif)ns of submarine cable 

 transmission have been concernefl principally with distortion and 

 means for correcting it. As terminal apparatus was gradually im- 

 proved means of correcting distortion were fleveloped which prac- 

 tically eliminated distortion as an important factor in the operation 

 of long cables. With ilistortion thus eliminated the speed was found 

 to be limited principally by the sensitiveness of the receiving ap- 

 paratus. This limit was, however, eliminated in turn by the develop- 

 ment of signal magnifiers. During recent years, in which numerous 

 cable signal magnifiers have been available and methods of correcting 

 distortion have been understood, the only factor limiting cable speed 

 has been the mutilation of the feeble received signals by interference. 

 Most cables are operated duplex, and in these the speed is usually 

 limited by interference between the outgoing anrl incoming signals. 

 In cables operated simplex, and also in cables operated duplex where 

 terminal conditions are unfavorable, speed is limited by extraneous 

 interference which may be from natural or man-made sources and 

 which varies greatK' in diU'erent locations. The strength of the 

 received current must in either case be great enough to make the 

 signals legible through the sujierposed interference current. Owing 

 to the rapidit> with which the received signal amplitude is decreased 

 as the speed of sending is increased, the limiting speed is quite sharply 

 defined by the interference to which the cable is subject. 



