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r.l.l.CI KIC.ll. TliSIS .1X1) nil. IK .{I'I'I.UJ I H)SS J55 



The testing circuits shown in Fij;. 1 arc comnionlN' used in tht; day 

 by day maintenance of the telephone plant. Resistance and resistance 

 balance measurements are nuuie periotiicalK' on (oil circuits to guard 

 ag;iinst series resistance imi)alances such as might be caused l)y high 

 resistance joints. The \'arle>' and Murra>' tests are constantly em- 

 ployed in directing linemen in clearing trouble such as crosses and 

 groimds. The Wheatstone bridge is therefore an important feature 

 of toll li-sl boards where ke\s are pro\ided to furnish a means for 

 ([uickK seltiiii; up llu' (iilTiTciit bridge lest circuit arrangements 

 desired. 



The \'arle\' or Murray tests used in connection with pole line 

 diagrams in locating troubles provide a means whereby the test !)oard 

 men can direct the movements ol linemen to the best advantage. 

 I nit resistance \alues with temperature corrections are available for 

 different types of circuits. If a gixxl circuit of the same type and 

 gauge over the same route is available, the unit resistance can be 

 determined directly by a loop measurement of this circuit. The 

 resistance values obtained by measurements on circuits having crosses 

 or groimds can then be used to determine the distance to the trouble 

 and the lineman sent to this point. By making measurements care- 

 fully and using the most accurate imit resistances available, troubles 

 can be located and cleare<l in the minimum amount of time. In 

 trouble location work on cables where the cable needs to be opened 

 to repair the trouble, bridge measurements are made to give the ap- 

 proximate distance to the fault. More e.xact locations can then often 

 be made by using an exploring coil test set by means of which the 

 cable repairman listens by induction to a tone sent out from the cable 

 terminal and determines in this way when he passes the point of 

 trouble. 



Leakaiie or Insulation Resistance Measurements. An important 

 factor in the maintenance of telephone circuits is to insure that there 

 arc no resistance leaks between conductors or Ixjtween conductors 

 .ind ground. It is also important to insure that insulated conductors 

 will not ha\e the insulation broken down by the voltages which are 

 met with under service conditions. Two types f)f tests now use<l 

 extensively in the plant are described below: 



(1) Voltmeter Method. This method is the one commoidx- used in 

 determining the leakage l)et\veen wires and to ground particularK' 

 1)11 toll circuits involving open wire and on subscribers' circuits. .As 

 >hown in Fig. 2 the testing arrangement consists of a Noltmeter in 

 series with a battery connected to the conductors under test. Diagram 

 (a) shows the connection for testing the leakage between wires and 



