3M 



BELL SYSTEM TECHNICAL JOURNAL 



current detecling ciexice, generally an ordinary telephone receiver. 

 Four important bridge measuring methods are used extensi\ety in 

 telephone testing work as described below: 



(1) Alternating Current Capacity Tests. The bridge circuit ar- 

 rangement for measuring a.c. capacity is shown in Fig. 10. 



Two arms of the bridge consist of fixed and equal resistances A 

 and B connected by a slide wire resistance, the position of the con- 

 tactor on this slide wire determining the total amount of resistance 



Kig. 10 



in each of the two arms. The fi.xed resistances in A and B are simply 

 extensions of the slide wire and can be cut out of the circuit when not 

 required. The third arm of the bridge consists of standard con- 

 densers C, and the fourth arm the circuit whose capacity Cx is to be 

 measured. A source of alternating current generally an 800 or 1,000 

 cycle o.scillator is connected to the terminals of the arms ^4 and B 

 while the telephone receiver is connected to the slide wire contactor 

 and to the junction of the standard condenser and circuit under test. 

 A balance of the bridge is obtained when there is minimum tone in 



the receiver, for which condition the common bridge formula Cx 



A 



C 



applies. The slide wire is caiii)rate(l to read the ratio B A directly. 

 For field testing work the above circuit arrangement is made up 

 in a portable box and a portable oscillator is used so that the apparatus 

 can be readily carried about as required. The commercial form of 

 bridge provides three values of standard condensers which can be 

 used to cover measurements from about .500 micro-microfarads up to 

 1.5 microfarads. This bridge linds its application in the plant in 

 measuring the capacity of short lengths of non-loadeil cable, bridle 

 wire, switchboard wire, etc. .Such measurements are of particular 

 importance in connection with the install, ii ion of 22 t\pe telephone 

 repeaters to determine llic iiropcr x.iiucs nl Imililing out condensers 

 to use in the line and b.ilancing circuits. 



