MEASUREMENT OF TELEGRAPH TRANSMISSION 159 



Fig. 10 — Stability test set with recording meter. 



recording meter. This set is also arranged for permanent mounting 

 on a relay rack in a central office. 



C. Portable Measuring Set for Use at Outlying Stations 

 A new measuring set has been made available for commercial use 

 especially at outlying stations in either routine or special testing. 

 This set is arranged for the accurate measurement of the systematic 

 components (bias and characteristic) of distortion of recurring test 

 signals. In addition measurements may be made of peak values of 

 interference and of the effect of bias and variations in operating 

 currents on the distortion. Such measurements are desirable in 

 analyzing the causes of transmission troubles and in equalization 

 work. By properly interpreting the results of these measurements a 

 fair idea may be obtained of the maximum total distortion. In 

 addition the set is arranged for the convenient measurement of the 

 operating currents and voltages in various parts of the subscriber's 

 circuit and in external circuits. This set may be used on either 

 110-volt a-c. or d-c. commercial power supply. It is mounted in an 

 aluminum case, and weighs only about 28 lbs. (13 kg.). 



The circuit employed for the measurement of systematic distortion 

 is indicated schematically by Fig. 11. This is a simple bridge type of 

 circuit similar to that generally used in a measurement of bias with 

 reversals but especially arranged to indicate directly the percentage 

 systematic distortion of the signals of Fig. 1 excepting " I " signal. The 

 meter circuit is highly damped to prevent undesirable vibration of the 

 meter needle in response to the 6-cycle fundamental frequency of the 

 60-speed teletypewriter characters. This entails the use of high 

 resistances R, and large capacitance C. Since the voltage in the power 

 supply is limited, a very sensitive meter M is required. 



