////• TR.INSMISSIOX IXIT .IXP RV.VI'.RHNCE SYSTEMS 401 



ri-artaiuv plarwl in serii-s iK-twwn the transmitter and the hne, may 

 ihange the (xiwer dehvered to the hne l)y the transmitter and hence 

 the oiit()ut of the receiver, the maijnitude and direction of the channt.- 

 iK'inj; determined by the impedance relatif)ns at the point of insertion. 

 The ratio of the jxjwer leaving the reactance to that entering it is, 

 of course, imity, as no jxmer is dissipated in a pure reactance. In 

 other words, the transmission efticiencN' of any part of a circuit cannot 

 he considereti solely from the standpoint of the ratio of output lo 

 input power for that part, or the power dissipated in that part, but 

 must l)e defined in terms of its effect on the ratio of output to input 

 (K)wer for the whole s\stem. 



By determining the effect of separaleh' inserting the main- pieces of 

 apparatus that may form parts of typical telephone circuits, an index 

 can Ih" estai)lishetl for each of ihe^e parts of its effect on the efficiency 

 of the circuit for the conilitions of which the circuit tested is typical. 

 Similarly, the power dissipated in unit lengths of the various types of 

 line can be determined b\- noting the change in power output of the 

 receiver caused by increasing an\- line h\ a unit length. Such indices 

 of the transmission efficiencies of the various parts of a circuit ob\i- 

 ously have many applications in designing and engineering telephone 

 circuits. These indices could be taken as the ratios expressing the 

 change in the output power of the system. This, however, has cer- 

 tain disadvantages. For example, the combined effect of a number 

 of parts would then f)e e.xpressed as a product of a number of ratios. 

 Likewise, for the case of a number of parts n of the same type in 

 series, such as a line n miles in length, the effect would be expressed as 

 the ratio for one part or one mile of the line, raised to the nth power. 

 In many cases, these ratios and the powers to which the>' would 

 need to l)e raised would be such as to make their handling cumber- 

 some. If, howe\er, these indices are expressed in terms of a loga- 

 rithmic function of a ratio selected as a unit, the sum of any number 

 of such indices for the parts of a circuit is the corresponding index for 

 the power ratio giving the effect of the combination of these parts. 



The "mile of standard cable" is such a logarithmic function of a 

 [xiwer ratio. The new unit also meets this important recjuirement. 



DlCIIMTIO.N OF THK Tr.VNSMISSION IMt 



The "transmission unit" (abbreviated TV) has been chosen so 

 that two amounts of power differ by one transmission unit when they 

 are in the ratio of 10°' and any two amounts of power differ by A' 

 units when they are in the ratio of lO-^'"". The number of trans- 



