THE TIME FACTOR IN TELEPHONE TRANSMISSION 55 



Since the speed of transmission in generally different for different 

 parts of the frequency range, for simplicity a particular narrow fre- 

 quency range, say between the frequencies .Vi and .^2, is considered. 

 It is supposed that electrical filters are applied to the circuit limiting the 

 frequencies over it to approximately this range. If then, a voltage 

 having a frequency, say at midpoint of this narrow range, is applied 

 to the circuit for a short interval and then removed, the speed at which 

 the disturbance thus set up travels down the circuit is the speed in 

 which we are interested. A spurt of energy of this type is evidently 

 similar to that which takes place in carrier telegraph systems when a 

 dot impulse is applied to the circuit. This speed can be looked at, 

 therefore, as that of carrier telegraph signals so formed. 



The speeds on this basis of a number of standard constructions which 

 represent good engineering practise today are approximately as follows: 



Approximate speed in 

 Type of circuit miles per second 



Cable circuits loaded with 88-mh. coils at 3,000-ft. spacing. . . . 10,000 

 Cable circuits loaded with 44-mh. coils at 6,000-ft. spacing. . . . 20,000 



Cable pairs of non-loaded 16 B. & S. gage 130,000 



Non-loaded open-wire pairs 180,000 



Radio 186,000 



Causes of Time Lag ix Transmission 



A pair of wires of zero resistance in free space separated from all 

 other conductors and without leakage would transmit electrical waves 

 over it at the speed of light. It will be noted from the above table that 

 non-loaded open wires transmit at a speed not differing widely from this. 

 What retardation exists comes largely from the glass insulators which 

 cause an increase in capacity and the resistance of the wires, which 

 causes an effective increase in inductance. 



In cable circuits there is still further retardation by the increase in 

 the capacity between the wires because of the necessity of using a 

 certain amount of solid dielectric and particularly from the increase in 

 the inductance of the wires when loading coils are inserted in them to 

 decrease attenuation. 



In actual circuits there is still some further retardation by the ap- 

 paratus which is necessarily inserted at the terminals and at inter- 

 mediate points along the circuit. The figures given in the above table 

 are for the bare circuits. The delays caused by apparatus will, in 

 general, reduce these speeds from 10 to 25 per cent. 



Slowing- Dow^N of Telephone Conversation 



Considering the first of the above factors, it is noted that so long as 

 the speaker at one end of a telephone circuit continues to talk, the 



