ELECTRICAL TESTS AST) TIIT.IR AI'I'LICATIOXS 



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jHMl.inre of niiuli-nsfrs iK-ini; lU'ij.itive reaclancc. In telephone cir- 

 cuits and etiuipnienl where inductance is involved such as in loading- 

 coils, transformers, retardation coils, etc., the effective resistance 

 as well as the inductance becomes a factor which must he taken 

 aci'ount of in brid^je testinj" work. For measuring effi-ctive resistance, 

 inductance and inifxtiance. bridges ha\e been developcfl which 

 are similar to capacity bridges except that standard condensers in 

 the balancing arm are replaceil by standard inductances and re- 

 sistances. 



There are two general tNjx^s of bridges in use in the telephone plant 

 designee! to measure imix-dance; one type for testing equipment made 

 up mostly of inductance, such as loading coils, and the other ff)r 

 testing the imr)edancc characteristics of various types of eciuipmenl 

 and circuits gencralK-'wiihin the operating range of frequencies. 



Recp 



The circuit arrangement shown in Fig. 15 is for an impedance 

 bridge designed primarily for measuring impedance of ecjuipment 

 having positive reactance characteristics. As in the capacity bridges 

 two arms are made up of fixed resistances A and B, connected by a 

 slide wire resistance. The imfjedance to be measured makes up the 

 third arm of the bridge and the standard impedance consisting of 

 known values of inductance and resistance is the fourth arm. To 

 obtain accurate measurements requires that the standard impedance 

 be approximately the same order of magnitude as the impedance 

 measured and the phase angles of the two must be very nearly the 

 same. X'alues of standard inductance are. therefore, chosen which 

 are known to \te fairly near the \alues of the unknown inductances 

 and a variable resistance R is provided which can Ik; switched in 

 series with either arm of the bridge and adjusted until the resistance 

 com{K)nents in the two arms are equal. The bridge is balanced by 

 adjusting the slide wire resistance and the resistance R until a mini- 



