Toll Switchboard No. 3 



By J. DAVIDSON 



IN the early days of telephony the toll signaling apparatus consisted 

 of a magnetic drop in the line and a drop or ringer in the cord. With 

 the advent of common battery signaling in the local plant, relays and 

 lamps replaced the old type drops and the subscriber was given means 

 for calling the toll operator on a toll connection by operating the 

 switchhook instead of ringing. Up to this time the toll operators were 

 located at the local switchboard and had direct access to the sub- 

 scriber's line, but with the growth of toll and local traffic, it was no 

 longer economical to place the toll operators at the local board. This 

 led to the development of a separate toll switchboard called the No. 1 

 board, which had access to the subscriber's line over switching trunks 

 between the toll and local boards. For many years the No. 1 switch- 

 board filled the needs of the time but with the expansion of the toll 

 service and the growth of machine switching local service, it became 

 evident that new arrangements were desirable. The No. 3 toll switch- 

 board was developed to meet the new requirements and it has the 

 following advantages as new installations are required. 



(a) Reduction in apparatus, resulting in equipment economies. 

 (&) Improved maintenance arrangements. 



(c) More readily adapted to modifications required by new operating 

 methods. 



In discussing the features of the No. 3 board, frequent comparisons 

 will be made with the No. 1 switchboard to set forth the changes which 

 have been made in the design of the new circuits. 



Main Features 



Cord Simplified by Locating Supervisory Relays in Line and Trunk 



Circuits 

 The cord circuit of the No. 1 switchboard is equipped with two 

 supervisory relays. One of these relays responds to 20-cycle current 

 and gives the toll operator a ringing signal, indicating that the distant 

 operator is calling. The second relay responds to direct current re- 

 ceived from the switching trunk and gives the operator switchhook 

 supervision of the subscriber. Associated with these two relays are 

 other relays which prevent false signals, and permit the operator to 



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