354 lU-LL SySTEM TliCIIMCAL JOVRSAL 



DlKIXT Cl KKKNT TeSTS 



The ti'sts imolxiiij; iht- use (if direct currents and \oltages pro\idc 

 means for checking some of the electrical characteristics of telephone 

 circuits and insuring to a certain extent that these circuits will give 

 satisfactory speech transmission. The application of these tests to 

 the telephone plant reduces to a minimum the amount of alternating 

 current testing required and lengthens tlu- inicr\a! at wliirli allcr- 

 nating current tests need be made. 



Wlieatstone Brid'^e Measurements. The various arrangements of the 

 Wheatstone bridge for direct current measurements and the principles 

 invoK'ed are well known and are therefore not discussed in an>- detail 

 in this pa[)er. Howe\er, due to the importance of such measurements 

 in tin- maintenance of telephone circuits and in trouble location work 

 a brief discussion of the general apjilications of the bridge is given. 



(c) Varley Loop to locale grounds (d) Murray Loop to locate grounds 



Fig. 1 shows four arrangements commonly t'mpl()\ed in muline 

 testing and trouble location. Diagram (a) of this figure gives the 

 bridge circuit for obtaining loop and single wire resistance measure- 

 ments. Diagram (b) shows the circuit for Varley loop measurements 

 to determine resistance unbalances in a pair when a third wire is 

 not available, while Diagrams (c) and (d) show thc\'arle\- and Murra\- 

 bridge circuits used in locating groimrls. Various other arrangements 

 of the bridge circuit are, of course, used where such arrangements will 

 facilitate the listing work. For a condition of l)ridge balance indi- 

 cated 1)>' no (leHeclion of the gaKanometer the \alue of the resistance 

 being measured is gixcn l)y ilu- well known bridge ratio formulae. In 



diagram (a) of l-"ig. 1. tor e\am|)ir. /^v= ,. K- 



