approaching as -an upper limit the velocity of propagation of light, 

 namely, 186,000 miles a vsecond. On loaded circuits this velocity is 

 greatly reduced. Loaded toll cal)le circuits in common use over 

 moderate distances, as already mentioned, have velocities of 10,000 

 miles a second and still lower velocities were associated with some of 

 the earlier types of loaded toll cable circuit. 



The transmission of telephone currents over long circuits is ac- 

 companied by reflections of a part of the current at points where the 

 electrical characteristics of the circuit change, particularly at the ter- 

 minals where it is not practicable to get a close match between the 

 characteristics of the toll circuit and of the various local circuits and 

 terminal equipments to which it must be connected. Considering for 

 the moment only this terminal reflection, speech over the circuit is not 

 only transmitted directly but also a part of the transmission current 

 is reflected back and forth between the terminals producing delayed 

 sounds analogous to the echoes produced w^hen one talks in the face 

 of a distant cliff or building. For example, over a 1000-mile circuit 

 with a velocity of transmission of 10,000 miles a second, the time 

 required for transmission in one direction is .1 of a second and for a 

 round trip of the circuit .2 of a second. On such a circuit the talker 

 may hear in his receiver the echo of his own words .2 of a second after 

 they are spoken and the listener at the other end may hear not only the 

 direct transmission but an echo delayed by .2 of a second. Such 

 effects, if sufficiently great, seriously interfere with the conversation, 

 the amount of interference increasing with the amount of delay. 



The reduction in the effect of echoes is partly taken care of by 

 improvements in the design, both increasing the velocity of transmis- 

 sion over the circuit and reducing the amount of reflected current. In 

 addition, special devices known as "echo suppressors" are used to 

 reduce further the effect of echoes. In the echo suppressor a small 

 part of the voice current is amplified and rectified and used to control 

 the circuit in such a way that during the conversation the circuit is 

 operative only in one direction at a time, the interruption of the return 

 path serving to prevent the transmission of echoes. As people speak 

 alternately from the two ends of the circuit, this control is automati- 

 cally shifted so that the words will be fully transmitted in each case. 



While echo suppressors are very successful and are widely used, they 

 have certain limitations and, in spite of their use, echoes remain an 

 important factor to be considered when engineering and laying out 

 long telephone circuits. 



Another eft'ect of the time of transmission arises from the fact that, 

 generally speaking, t'.e components of different frequencies making 



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