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BELL SYSTEM TECHNICAL JOURNAL 



curate adjustment of the sensitivity to compensate for small differences 

 between instruments and any slight changes which may occur with 

 time. The particular arrangement shown in the diagram has an input 

 impedance of about 7500 ohms and a range of + 4 to +26 vu for 

 readings at the vu or 100 mark on the instrument scale. 



Diagram 18B shows a low impedance arrangement in which by 

 adding a transformer the sensitivity has been increased by 10 vu at the 

 expense of decreasing the input impedance to 600 ohms. The circuit 

 is so designed that the impedance facing the instrument is the same as 

 in diagram A, so that the proper dynamic characteristics are obtained. 

 This arrangement, being low impedance, cannot be bridged across a 

 through line, but must be used where it can terminate a circuit. It is 



AMPLIFIER 



PROGRAM 

 LINE 



\> 



ANV 



PROGRAM LINE 



-VAr 



■A/vV 



-vw- 



PROGRAM LINE 



-VvV 



AW 



VOLUME 

 INDICATOR ' 



Fig. 19 — Program bridge for feeding several lines from one line. 



useful for measuring the transmission loss or gain of a circuit on sine- 

 wave measuring currents, and also for measurements of volume level 

 where it is connected to a spare outlet of a program bridge crcuit, as 

 shown in Fig. 19. Program bridge circuits, one form of which is 

 illustrated in the figure, are commonly employed in the Bell System 

 when it is desired to feed a program from one line simultaneously into 

 a number of other lines. The bridge circuit which is illustrated consists 

 of a network of resistances so designed that the volume level into each 

 of the outgoing lines is the same, that the impedance presented to 

 each is the correct value of 600 ohms, and that the attenuation through 

 the network between any two of the outlets is great. 



A picture of a volume indicator which is provided with both of the 

 circuits shown in Fig. 18 is illustrated in Fig. 20. 



