ELECTRICAL TESTS .IXD THEIR .irri.lC.ITIOXS 379 



with tlu- "il-cirruit tt-sts, this is an indiiatioti of halancf troiiblr vvliich 

 imisl hi' lotatcil t-ithcr by an inspoition of tin- riiciiits or lialanrinp 

 eqiiipint'nt or by rt-sortin^j to inipi'daiu'c iiUM>iiifmfiits as di'scrihi'd 

 previously. 



Anothi-r mcth(xl of di-tt-rnunini; im(K'dance irregiiiaritii-s which 

 is maik' use of in sonic of the larger oltices is to measure the trans- 

 mission loss through the 3-wiiuiing tr.msformer with the hnes and 

 networks connecteil as for normal repeater operation. As stated 

 previously the loss tlirough the transfornier to currents from the 

 output of one ampliher to the input of the other gives a measure of 

 the balance conditions of the line and network, the loss increasing as 

 the balance becomes more perfect. By this scheme the losses through 

 the 3-wintling transformers can be measured ovei a range of frequencies 

 as in line impedance measurements and a loss curve obtained which 

 can be used to locate irregularities in the same manner as described 

 for line impedance curves. 



Transmission Efficiency Measurements. If all or a part of the tests 

 already described were applied to the various transmission circuits 

 in the telephone plant, most troubles which might effect speech 

 transmission could be delected and assurance given that the circuits 

 were projx^rly installed. Such a procedure would be costly and im- 

 piacticable and for this reason it is necessary that means be provided 

 wheieby a measurement of a circuit's efHciency for the transmis- 

 sion of voice currents can be quickly made. 



The transmission of voice currents can be measured in terms of a 

 standard and expressed in units in much the same manner as the 

 transmission of any electrical currents. A telephone circuit, for 

 e.xample, extending between any two ofifices is said to have an equiva- 

 lent of so many units of transmission, the number of these units de- 

 pending on the electrical characteristics of the component parts of 

 the circuit." Transmission measurements, as far as volume etificiency 

 is concerned, involve determining by means of suitable testing ap- 

 paratus the number of transmission units of loss or gain which a 

 particular circuit or piece of equipment causes. As it is desired to 

 obtain a measure of efficiency at a frequency comjiarable with the 

 combinetl frequencies of the voice, a frequency of 1,000 cycles for the 

 testing current has been chosen which experience has shown gives 

 results approximating fairly closely those (obtained by using a com- 

 bination of the fre(|uencies within the voice range. Measurements 

 can also be made at other frec|uencies within the voice range or at 



' See the article in this issue. The Transmission Unit and Telephone Transmission 

 Reference Systems, by VV. H. Martin. 



