370 



BELL SYSTEM TECHNICAL JOURNAL 



imim loiu' i^^ lu'ard in the rcci-ixcr. For llio condition shown in l-is^. l.") 

 wln'n tin- hridin' is balancetl L, = W ^s and Rx='^ {R + R,). The 



B 



B 



shdi- wire is cahbrated to read the ratio — - directly and tables of values 



3 



for L5 and Rs at various frequencies are supplied for use with the 

 roniniercial form of bridges. The \aluc of R is read dirertK' from the 

 dial rheostats on the bridge. 



In practice this form of bridge finds its principal ajiplication in 

 measuring the inductance and resistance of cable loading coils when 

 trouble is experienced which necessitates opening up the cable and 

 loading coil pots. It is also used to measure the unbalance between 

 windings of coils as, for example, between the line windings or the drop 

 windings of repeating coils. For measurements of the latter kind 

 one winding is connected in place of Lx and the other in place f)f Ls 

 and the unbalance between the two w-indings is then given by the 

 slide wire ratio. A further use of this scheme is in checking the cor- 

 rectness of loading of short cable circuits and a special bridge has 

 been designed for this purpose. A pair which is know'n to be properK' 

 loaded is used as the standard and ail other pairs of the same lengtii 

 and loading are checked by connecting them one at a tinK- inio ilu- 

 iniknown arm of the bridge. 



Circuit under Test 



K. 16 



Tlu- form (it bridge designed to nicasurc ihc impedani'e character- 

 istics of circuits and equipment at aiu desired fre<|uenc\- or at a num- 

 ber of frequencies is shown in Fig. Ki. 



The fixed resistances A and B, geiur.ilK of !,()()() ohms each, make 

 up two arms of the bridge, the circuit under test the third arm and a 

 variable resistance and a variable inductance standard the fourth 

 arm. The variable inductance L is arranged so that it can be switched 

 in series w'ith the circuit under test when the characteristics of this 

 circuit are such that its capacitive reactance predominates. I'or a 



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