24 BELL SYSTEM TECHNICAL JOURNAL 



This circuit arrangement not only effects substantial economies but it 

 is much more flexible and will lend itself to new developments without 

 requiring changes in the cord circuit. 



Monitoring and Ringing Keys Individual to Cords 

 The monitoring and ringing keys are, as in the past, individual to 

 each cord. 



Switching Trunk Features 

 In the No. 3 toll switchboard a repeating coil which has a high im- 

 pedance to 20-cycle ringing current is used in the outgoing end of the 

 switching trunk. This arrangement has equipment and signaling 

 advantages. Also where loaded toll switching trunks are involved, the 

 use of a repeating coil of the type referred to, but having the proper 

 transmission characteristics, has the advantage of reducing reflection 

 losses by providing for a uniform terminal impedance of the switching 

 trunks. 



Principal Advantages 



Equipment Economies 



As has been pointed out, the expansion in toll business, together with 

 recent developments in the telephone art, have been such that with the 

 circuit arrangements used in the past there has been a growing necessity 

 to add equipment to the cord circuit with the result that the positions 

 are becoming congested with apparatus. With the circuit arrange- 

 ments outlined for the No. 3 toll switchboard, however, the transfer of 

 the signaling apparatus from the cord to the line and switching trunk 

 makes a marked simplification in the cord and incidentally reduces the 

 congestion in the section. Also it should effect a substantial economy 

 in equipment because of the fact that we are approaching a situation 

 where there are approximately 60 per cent, more cords than lines and 



25 per cent, more cords than switching trunks. 



The use of the positional circuit and the elimination of the in- 

 dividual splitting key from the toll cord has simplified the switch- 

 board keyshelf. This simplification together with the equipment 

 savings effected by the simplification of composite ringers and the 

 transfer of the supervisory relay equipment from the toll cord to the 

 toll line and switching trunk circuits has effected substantial economies. 



Maintenance 

 In addition to the saving in first cost of equipment the No. 3 switch- 

 board facilitates maintenance. The ordinary toll cords in an office 



