ELECTRIC CIRCUITS APPLIED TO COMMUNICATION 17 



suppressors are used to intercept the echo currents and prevent their 

 interference with the speaker. 



Another important effect is the imperfect equaHzation of the trans- 

 mission of different frequencies within the range of important telephone 

 frequencies. This imperfection can be offset by the use at intervals of 

 correcting networks which introduce a distortion opposite to that 

 produced by the line, and the design of such networks is one of the 

 interesting problems which has been worked out in connection with 

 these very long circuits. The distortion is not only one of magnitude 

 but also one of phase due to the difference in the velocity of wave 

 propagation of component currents of different frequencies. This may 

 be extremely important on long circuits of the heavier type of loading 

 in which any minor disturbance at the transmitting end of the line is 

 transmitted in such a way that the low frequency components appear 

 first at the receiving end, followed by progressively higher frequency 

 components and causing disturbing transient noises somewhat similar 

 to the chirrupping of a bird. Phase distortion is less on the more 

 lightly loaded circuits but still remains of enough importance to require 

 the use in some cases of networks to equalize the distortion of phase. 



In this discussion of very long circuits I have talked of telephone 

 circuits. In the case of both telephone and telegraph circuits the 

 fundamental requirements are the same, namely, the propagation of 

 currents within a certain range of frequencies without excessive dis- 

 tortion and without interference from other electric circuits. The 

 principal difference in the problems is in the range of frequencies which 

 is important in the two cases, that for telegraph being much lower 

 and more limited in extent than that for telephone. Another difference 

 is that in the case of telephony phase distortion is important only in 

 producing different time of arrival of different components, whereas 

 in telegraphy the effect of phase distortion in distorting wave shape is 

 also of importance. The telegraph problem can, like the problem of 

 transients in telephone lines, be approached theoretically from the 

 performance of a circuit when a potential is applied suddenly at one 

 end. It has been shown, however, that the treatment of the circuit 

 in terms of its steady state characteristics for the propagation of al- 

 ternating currents over a range of frequencies leads to results identical 

 with those reached by the transient treatment, and for most cases the 

 steady state method of treatment rather than the transient method of 

 treatment is found to be more convenient to handle for purposes of 

 circuit and apparatus design. 



