PRACTICAL APriACATlOX 01- THE TRAXSMISSIOX CXIT 411 



fri'tiiu'tiry is 0.11."). The ratio of the elTect on voiimic of tlic luilu of 



122 

 ^taiulanl cable to tl»e TU is, therefore, '.-r. or 1.0(5, and equivaicntfc 



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obtainetl by comparison with standard cal>le by means of talkini; 



tests can therefore be con\ertod to 77' in' miiltipiyinj; !>>• tliis factor, 



as previously indicated. 



The SOO-cycle mile whicii has been commonly used in e.xpressinj; 



computed transmission losses, has an attenuation of 0.10!) to currents 



of any frequency, and therefore data expressed in 800-cycie miles are 



converted to TU I y niultiplving bv / , ' or 0.95. 



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For making talking tests the field has been supplied with arliticial 

 cables which were slightly different from standard cable, having a 

 capacity of Mfif. per mile instead of .054 /jf- Miles of this artificial 

 cable may be converted to TU by multiplying by 1.12. 



The conversion of subscribers' loop losses to TU is somewhat more 

 complicated as the zero of reference for subscriber's set efficiencies 

 is slightly different on the new basis. In the Bell System, therefore, 

 complete data on subscribers' loop losses in terms of the new unit were 

 made available for engineering work at the time the TU was adopted. 



The transmission equivalent of a line per unit of length in TU may 

 be obtained by multiplying the attenuation constant of the line 

 computed in the usual manner by a conversion factor. Calling the 

 computetl attenuation constant of the line per unit of length a, the 



number of TCwill be given bv the expression: TU= = 8.69a. 



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In finding the total loss which a short line or a piece of equipment, 

 such as, for example, a repeating coil will cause when inserted in a 

 given circuit, the current in the receiving apparatus is usually com- 

 puted for a convenient voltage applied at the sending end of the 

 circuit, first with the repeating coil in the circuit and then with it out, 

 the applietl voltage remaining constant. Calling these currents 



/i and /j respectively, the current ratio —! may be converted into 



TU by the expression 



Loss in rL' = 20 logio y. 



Tr.\nsmission M.\intenance 

 The transmission measuring sets used for checking up the main- 

 tenance ' of the plant from a transmission standpoint, have previously 



' See an article in this issue " Electrical Tests and Their .Applications in the Main- 

 tenance of Telephone Transmission" — \V. H. Harden. 



