380 



BELL SYSTEM TECHNICAL JOURNAL 



frequencies oul-side of this range where desired, for example, at ringing 

 current frequencies or carrier current frequencies. 



KfticiencN' tests of transmitters and recei^■ers present a somewhat 

 different problem and for these it has been found most convenient to 

 make direct comparisons between the instruments under test and 

 standard instriniients. 



A discussion of the ajiplicalion of transmission testing ai)paratus 

 in maintenance work for measuring losses and gains is gi\en below. 



(1) Measiirenients of Transmission Losses. In its simplest form a 

 transmission measuring set involves an arrangement of apparatus 

 whereby a volume comparison can be made between \oice currents 

 transmitted over a circuit of unknown efficiency and then o\er a 

 standard circuit of known etificiencv. 



Kig. 23 



.Sucli an .irniiiKcniciit is ijliistrali'd in F'"ig. '2'.\ In wliicli ilic anunint 

 of artilicial ciblr rcqnii-cd to give a volume of transmission i'(iiial 

 to that oiilaiiuil iiNir ihr i-ircuil under test is a mcasini' nl llic i in nit's 

 cftiricncN- in Urni> nl I hi' artificial cable units. Prior to the develoj)- 

 nicni of ilic piv-iiit i\ pi> of transmission measuring sets the arrange- 

 ment shown in I'ig. 2,i was used to a limited extent, principally in 

 making measurements on important types of toll circuits and in 

 determining fundamental transmission data such as imit equivalents, 

 rellection losses, etc. 



To nu-el the practical ii(|iiirtnunts of field testing work two general 

 t\|)es of Icsiiiig .i|)p.uatns haw bci'ii di;\elope(l, oin' in\-olviiig "ear 



