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BELL SYSTEM TECHNICAL JOURNAL 



quency field intensity measuring unit ^^ is a convenient indicating device 

 and source of signal. The intermediate-frequency amplifier with its 

 adjustable gain is very useful in maintaining a desirable level in the last 

 detector which is the indicator. Returning to Fig, 16, the local signal 

 oscillator is the source of voltage actuating the tuned circuit LC1C2 in 

 which the substitutions are made. Loose coupling is desirable between 

 the pick-up coil and oscillator. In measuring Zi, which is a low im- 

 pedance, the condenser d is set at minimum capacity thus effectively 



Fig. 17 — Experimental set-up of apparatus for measuring line loss. 



making the comparison in a series circuit. In measuring Z2, which is 

 of the order of hundreds of ohms, C2 is used to transform Z2 into an 

 appropriate series value consistent with selectivity and sensitivity. 



The comparison resistances used for measuring (Zi) may be fixed 

 units and the line impedance obtained by interpolation. These com- 

 prise straight lengths of high-resistance wire and range from a few 

 tenths of an ohm to ten ohms. The wire size is chosen so that skin 



^' Readers not familiar with this measuring unit may refer to a paper by Friis 

 and Bruce, Proc. I. R. E., August, 1926. 



