////•: IR.IXSMISSIOX rXIT .I.V/) KV.I-F.RENCE systems 40.1 



For a given loiuliioss of sjx^^ch souiuls cntoring the transmitter at 

 one end of the circuit, tiie louilness of the reproduced sounds given 

 out by the riTeixer at the other end can be varied by ciianging the 

 amount of standard cable in the circuit. Also, the amount of cable in 

 the circuit can Ix; used to express the ratio of the power of the re- 

 proflucetl sounds to that of the impressetl sounds. Due to the dis- 

 sipation of electrical power in the cable, this ratio and consequently 

 the loudness of the reproduced sounds become less as the amount 

 of cable is increased and greater as the length of cable is decreased. 



This circuit then became the measuring or reference system for 

 engineering the telephone plant and the "mile of standard cable" 

 iK-came the unit in which the measurements were expressed. This 

 circuit was usetl to set the service standards in designing and laying 

 out the telephone plant. Thus, the reproduction obtained over this 

 circuit with a length of cable of about twenty miles was found suitable 

 and practicable for local exchange, that is, intra-city service, and that 

 corresponding to about thirty miles for toll or intercity service. 



Any telephone circuit was rated by its comparison with the standard 

 circuit. This comparison was on the basis of a speaker talking alter- 

 nately over the circuit to be measured and the standard circuit and a 

 listener switching similarK' at the receiving ends, the amount of cable 

 in the standard circuit being adjusted until the listener judged the 

 volume of the sounds reproduced by the two systems to be equal. The 

 number of miles of cable in the standard circuit was then used as the 

 "transmission equivalent" of the circuit under test. The effect of 

 any change in the circuit under test on the efficiency of that circuit 

 could then be measured b>- determining the variation in the amount 

 of standard cable required to make the sounds reproduced by the 

 two systems again equal and the number of miles of standard cable 

 reciuired to compensate for this change was used as the index of this 

 effect. In this way the relative efficiencies of two transmitters or 

 receivers could be determined. Likewise, the power dissipation per 

 unit length or the attenuation, of the trunk in the circuit under test 

 could be equated to miles of standard cable. Since in each case, 

 the standard cable is used to adjust the volume of the reproduced 

 sound, "the mile of standard cable" corresponds to the ratio of two 

 amounts of sound jwwer, or as this change in sound power is pro- 

 duced by changing the power deli\ered to the telephone recci\er, to a 

 ratio of two amounts of electrical power. 



If the addition of a mile of standard cable to a long trunk of the 

 standard circuit causes the power reaching the end of the trunk to 

 decrease by a ratio r, then the insertion of two miles will decrease 



