INTERFERENCE ON SUBMARINE TELEGRAPH CABLES 415 



adaptable. Fig. 6 shows the real and imaginary parts of the character- 

 istic impedance of a tyi)ical cable designed to operate at a speed cor- 

 responding to about 60 c.p.s. It is evident that for all frecjuencies 

 above 20 c.p.s. the impedance can be approximated very closely by a 

 pure resistance of about 400 ohms. In contrast to this, the character- 



10 



20 



80 



90 



30 40 50 SO 70 



FREQUENCY- CYCLES PER SECOND 

 A. Resistance Component. B. Reactance Component (negative) 



istic impedance of a non-loaded type of cable varies with frequency 

 and has a reactance component about as large as the resistance com- 

 ponent. In the case of the loaded cable the problem of designing a 

 terminating network for the sea earth conductor is therefore com- 

 paratively simple, being a matter of finding a method of including 

 in the cable structure a resistance of several hundred ohms. It is true 

 that a network of this sort does not provide a good balance for fre- 

 quencies much below 20 c.p.s., and components of interference of these 

 low frequencies will be found at the cable terminals due to the lack of 

 balance between the main cable and the sea earth. As was previously 

 pointed out, however, these components will be so greatly attenuated 

 by the signal correcting networks that their effect upon the receiving 

 instrument will be inappreciable. This is illustrative of a general 

 property of the loaded telegraph cable, namely, that when a cable is 

 suitably designed for the frequency at which it is to be operated its 

 characteristic impedance approximates closely to a resistance over a 



