Crossbar Dial Telephone Switching System * 



By F. J. SCUDDER and J. N. REYNOLDS 



This paper describes the crossbar dial telephone switching system 

 recently adopted by the Bell System for large cities where the panel 

 system has been used for nearly twenty years. Central offices of 

 the crossbar type can be introduced in panel areas without changes 

 in existing offices and without changes in existing dial telephone 

 instruments. Crossbar offices and panel offices in the same build- 

 ing will operate on a common power plant and utilize other equip- 

 ment in common, such as "A" and " B " operator switchboards and 

 outgoing trunks. 



The precious metal contact crossbar switches are used for all 

 switching purposes as contrasted with the base metal contact panel 

 switches. The switches operate with relay-like movements under 

 control of common control or marker circuits consisting primarily 

 of multi-contact, U and Y type relays. The control and marker 

 circuits, which are connected to the switch frames by means of 

 multi-contact relays, perform their operations in a fraction of a 

 second. The switches, the U and Y type relays and the multi- 

 contact relays are equipped with twin contacts of precious metal. 

 Senders similar to those of the panel system are employed. 



The system will be used for new offices in larger cities as manu- 

 facturing and plant conditions permit. 



Introduction 

 TT is the purpose of this paper to describe briefly the crossbar dial 

 ^ telephone central office switching system which has recently been 

 developed by the Bell System for use in large cities. Sixteen years 

 ago, in February 1923, a paper was read before the Institute, by 

 Messrs. E. B. Craft, L. F. Morehouse and H. P. Charlesworth of the 

 Bell System which outlined the history and the problems involved in 

 telephone central office switching and described the panel dial central 

 office system which had just been developed and was being introduced 

 in the large cities. The first central office of this type was placed in 

 service in December 1921, and since that time 456 panel dial offices 

 serving nearly four and one-half million subscriber stations have been 

 installed in 26 different cities throughout the country. During these 

 years many improvements have been made in the panel system to 

 make it more serviceable to the telephone public and to meet the new 



* Presented at Winter Convention of A.I.E.E., New York, N. Y., January 23-27, 

 1939. 



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