54 BELL SYSTEM TECHNICAL JOURNAL 



under some conditions to hold circuits stable, it has become the prac- 

 tise of connecting into certain types of circuits, relay devices operated 

 by the transmitted speech currents which render inoperative trans- 

 mission in the opposite direction. In some cases delay in transmission 

 may be an advantage in the operation of such devices. In other 

 cases it may introduce serious difficulties. Conditions may be set up 

 in which it is difficult for one party to interrupt the other. In other 

 cases, portions of conversations may be locked out. If the voice- 

 operated devices are not properly adjusted or if considerable noise is 

 present, the devices may not function properly and speech mutilation 

 may result. 



5. Fading. In radio, the well-known phenomenon of fading is 

 due to waves arriving at the receiver over different paths, the trans- 

 mission times of which are such as to cause alternate strengthening and 

 weakening of the received signal by alternate phase agreement and 

 opposition. While this factor is mentioned here for completeness, it will 

 not be discussed further as it is beyond the scope of this paper to discuss 

 the problems introduced when there is more than a single path between 

 the sending and receiving ends of a circuit. The present paper is 

 limited to the conditions which hold where not more than one path is 

 involved in the transmission in each direction. 



Speed of Transmission 



Before considering these problems in more detail it would be well to 

 define what is meant by the speed of transmission over a circuit. 

 There are several speeds which may have significance according to the 

 problem involved. 



Whenever a change in applied voltage is made at one end of a circuit, 

 some evidence of this is transmitted over the circuit to the receiving 

 end at the speed of light. In general, however, except in radio, no 

 sufficient action to be of use is transmitted at this speed and it is largely 

 of theoretical importance. 



The speed which the engineer generally has in mind in thinking of 

 line transmission is the speed at which the crests or the troughs of the 

 waves pass along the line when a single-frequency potential is contin- 

 uously applied at the sending end. This usually is referred to as the 

 speed of phase transmission in the steady state. While this usually 

 approximates the speed in which we are interested, it may in particular 

 cases differ considerably from it. In fact, in certain types of artificial 

 circuits the crests and troughs of the waves travel toward rather than 

 away from the sending end. 



This speed may best be explained as follows: 



A 



