The Bell System Technical Journal 



Vol XVII January, 1938 No. 1 



New Transmission Measuring Systems for Telephone 

 Circuit Maintenance 



By F. H. BEST 



Transmission measurements on telephone circuits have long been 

 recognized as essential aids in the furnishing of good service. Re- 

 cent development work has produced new testing methods and 

 apparatus which greatly simplify and expedite transmission meas- 

 uring. This paper gives a brief description of the existing and the 

 new arrangements. 



ABOUT two decades ago pioneering work was being done in the 

 development and introduction of methods and apparatus for 

 the measurement of the transmission characteristics of local and toll 

 telephone circuits, it having been realized that with the increasing 

 complexity of the telephone plant, these measurements were necessary 

 to insure satisfactory transmission performance. Transmission meas- 

 urements now have a well established place in maintenance activity 

 and development work is constantly in progress to improve the testing 

 apparatus so that it can be operated more rapidly and conveniently, 

 will cost less or do more things, and also to improve testing methods. 



The principle employed in making transmission measurements has 

 not changed. A standard testing power is supplied to one end of a 

 telephone circuit and the power received at the other end is measured. 

 The ratio between these powers, expressed in decibels, is a measure of 

 the transmission loss or gain in the circuit. The magnitude of the 

 testing powers is small, the sending power for measuring being one- 

 thousandth of one watt and the received power ranging from one 

 ten-thousandth to one-millionth of a watt. Sensitive meters must be 

 used, or the power amplified before measuring it. Direct-current 

 meters are more sensitive than alternating-current meters and it is 

 the practice to convert the weak received alternating current to direct 

 current by means of vacuum tube or copper-oxide rectifiers and employ 

 direct-current meters for measuring it. 



Until recently the developments have been along the line of im- 

 provements in the apparatus itself, there being practically no change 



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