24 BILU. SYSTEM rr.CIIXlC.U. JOrRX.IL 



fcrring llu' start li-ad to the ln\v<.r relax' f)f ihv third jLiroup wliiiii 

 operates when the start lead is again grounded through the make 

 contact of the stepping relay. This cycle is repealed for each seg- 

 ment which the commutator Ijrush passes over until the upper rela\ 

 of the fourth or zero group of counting relays operates and opens thi' 

 fundamental selecting circuit, thus allowing the line relay in the tinal 

 frame to release when the commutator brush again removes the shunt. 

 The line rela\', on releasing, opens the up-drive circuit and the selector 

 stops with the multiple brush resting on the particular terminal 

 desired. 



There are three differeiii tyjjes of rela\s in this circuit, 'llie line 

 relay on the final frame is the general utility punched-t\pe relay of 

 Fig. 13 with the contact spring assembh- and mechanical adjustments 

 required by the specific circuit cniiditioii. It is exident that this 

 relay must release quickly enougli to eiuilik- the U])-(lriM' clutch 

 magnet to release before the selector is driven bevond the desired 

 terminal or a false bank terminal selection will be made. .An exam- 

 ination of some of the factors inlluencint; tlu- release time of the line 

 relay will therefore be of interest. 



When the commutator \)vu>\\ made ((inlacl with tlu- inmnuitator 

 segment both ends of the inter-olVue tiinik were grounded Imt before 

 the brush left this segment the conden.ser charge on the trunk leads 

 was dissipated and the distant end of the trunk was opened b\- the 

 operation of the iijiprr counting relay of the zero group. On K.ixing 

 the fourth i(iniiniii,iinr segmeiil the i)rush opened the circuit of tlu- 

 line relay which cnuld not release instantaneously because of its own 

 time constant, the transient current through its windings for charg- 

 ing the trulik capacit\ , and the leak current in its windings resulting 

 from trunk leakagi'. 



The time constant is determined by the electrical and magnetic 

 constants of the rela\- and for a given winding is inherent to its struc- 

 ture. If the time constant is such that adjustments, for armature 

 air gap, spring tension and contact separation, cannot be made which 

 will enable a rela\- to meet all the circuit requirements, .i dilTereiit 

 t\pi' of rela\- structure ha\ing .i more l,i\i)r.ilile time constant nuist 

 be used. 



The magnitude of the ch.irging current lor the trunk is (k-ti'rmined 

 li\ ihr trtmk ca|)acity and is in direct |)roiK)rtion to the length of the 

 trunk which is limited to 12 miles corresponding to a ina.\imiim 

 capacity of about ().S4 mf. The limiting open circuit resistance of 

 the Innik is :{l),(ll)() ohms and the standard of maintenance is such 

 til, It the in>ul.ili"n resistance is not allowed to drop below this \alue. 



