I i.iAi Kh.ii. iiisis .t\i> nil ih: .ifi'i.n.iiioxs .i5g 



Current h'hm: and \'oltai^f Measurenifnls. Ti'sis |o (Irii-riniiu- ihc 

 amount of <liriHt rurri-nl llowiii^; ii) idi-phoiu' rimiits iiuoKc ilu- 

 simpli" arraiiijtMiiciU of an ammeter or milli-amiiu'liT in series with a 

 (I.e. generator or liattery and the eircuit uiuler test. The amount of 

 current flowing; is, of eourse, a function of the resistance of llie circuit 

 .ind the voltajje applied. In tesliiiK arrangements where it is neces> 





Fig. 5 



3 





•<\ry to know the hatter>' or generator potential, voltage readings are 

 made by using ordinary voltmeters ha\ing the prt)per range and 

 resistance. Direct current and voltage measurements can best be 

 described by considering two of their applications in the telephone 

 plant. 



Fig. 5 shows a simple test circuit proxide.l in llu- 1 ical test desk 

 wherein- central office battery is supplied through a regulating rheostat , 

 a standaril cord circuit and a meter. Knowing the voltage ol the 

 central office battery and the resistance in the test circuit, the reading 

 of the meter when circuits such as a subscriber's loop or trunk con- 

 ductors are connected and shorted at the distant end gives a means 

 for determining the direct current resistance of these. Tables are 

 generally providetl for use at the test (.lesks by means of which different 

 readings of the meter for difTerent conditions of measurement cm be 

 converted directly into resistance values. The rheostat in the test 

 circuit .is pro\ided primarily for adjusting the current supplied to 

 subscribers' loops and instruments to the same value for different 

 lengths of loop. Talking tests as mentioned later in connection with 

 substation maintenance can then be made from the instruments to 

 the test man in the central office under the same current supph' con- 

 ditions for different lengths of loop at the time substations are in- 

 stalled or when these are reported in trouble. The arrangement shown 

 in Fig. 5 is useful in detecting high resistances in circuits when a 

 W'heatstone bridge is not available. High resistances in the main 

 frame protector springs and heat coils of lx)th subscribers' lines and 

 toll circuits are also determined by a current How methcKl, a special 

 fK)rtable testing set, however, being designed particularK' for this 

 piir[x>se. 



