IXTEUTOLL TKUNK CONCENTKATIN'C KQUIl'MEXT 317 



or highost iiiioperated or relay in the group dependiug on the direction 

 of selection. If the path is found to Ix^ closed to ground the ss relay will 

 ()p(M"ate. Tht> \s relay is a polar n^ay. 'fiie ixvsistance shown in series 

 with the secondary winding will have a \alue depending upon the num- 

 her of select magnets which are multipled ti)g(ither on each Unci of the 

 l)arti(ular concentrator, which number is a function of the number of 

 incoming trunks. The xs relay will operate on every normal connection 

 since the ciu'rent in the s winding, which is in the direction to operate 

 tli(> rcla}', will be larger than that in the primary winding. When the 

 resistance to ground on the select magnet lead is less than it should be, 

 due to a cross with another select magnet lead or to a direct ground, the 

 current in the primary winding will be greater than that in the secondary 

 winding and the resultant ampere turns will be sufficient in the non- 

 operate direction to prevent the operation of the xs relay. This would 

 cause the selection to be halted, the controller to time out and the trouble 

 registered. A reversal in the direction of selection would occur and selec- 

 tions would then be resumed. If no fault is found with the select magnet 

 operating path the select magnets operate. The controller then introduces 

 a small time interval to permit all parts of the selecting bar operated by 

 the select magnet to come to rest. The hold magnet operating path is 

 then closed as discussed below^ and the crosspoint is closed. The closure 

 of the crosspoint operates a relay os in the outgoing trunk which in turn 

 releases the select magnets and ss relay which releases the xs relay. 



ffoJd Magnet Operating Circuit 



The hold magnet operating circuit is shown in Fig. 8. Two circuits are 

 provided, one for each direction of selection to insure against blocking in 

 case a trouble in this portion of the controller. The cross detection part 

 of this circuit is an unbalanced wheatstone bridge, the galvanometer 

 element of which is the polar relay xh. Three of the arms of the bridge 

 are resistances, the values of which are tailored to each particular con- 

 centrator depending upon the number of hold magnets to be encountered 

 on a normal connection. This number is a function of the number of 

 outgoing trunks. The fourth arm consists of the hold magnets. When the 

 TENS GATES relays released as previously described the xh relay operated 

 (at this time the hold magnets are not connected and the bridge is not 

 foimed). Later in the progress of the call, when the units relays operate, 

 the hold magnets are connected and the bridge is formed. If th(> rc^sistance 

 (»f the hold magnet arm of the Ijridge is as expected, the bridge is un- 

 l)alanced so as to keep current flowing through the xh relay in the direc- 

 tion to maintain it operated. This will permit a relay ht, which furnishes 



