The Bell System Technical Journal 



January, 1924 



Relays in the Bell System 



By S. P. SHACKLETON and H. W. PURCELL 



Note: Before they can converse people must either be brought together 

 or virtually be brought into one another's presence by the telephone. Any 

 telephone system must establish talkinir connections between its subscribers, 

 and these connections must be built up, supervised and disconnected when 

 desired. This work is accomplished by the use of relays of various kinds, 

 and the speed and accuracy of service is largely dependent upon them. 

 There are completed daily in the Bell System about 42,()()(),()()U telephone 

 calls. These involve the successful and accurate operation of over one and 

 one-half billion contact connections daily. 



Many kinds of relays are employed in the Bell System, var>'ing from the 

 simple electromagnetic drop to the sequence switch, the thermionic vacuum 

 tube and the panel selector. Toda\' a circuit connection between two 

 subscriliers served by manual exchanges in a large multi-office district 

 invoUes about 21 relays. When these subscribers are served b)- machine 

 switching offices, the number of relays in a local connection may be as 

 great as 146. It not infrequently ha; n-ns that in setting up a toll con- 

 nection more than v^OO relays are empK -H. 



In the present paper the relay developments leading up to, and making 

 possible the present communication system, are outlined with particular 

 reference to electromagnetic relays. A few typical circuit applications are 

 given with a discussion of the requirements imposed upon relays which 

 influence their design. Several types of relays are illustrated and their dis- 

 tinctive features are described. 



The subjects of relay design, manufacture and maintenance and also 

 telegraph relays will be dealt with in future papers. 



Introduction 



IX the vast systems of networks \vhit;li comprise the Bell Sys- 

 tem one of the most important and varied devices necessary 

 for giving service is the relay. From its use in small numbers 

 in telegraph circuits and as a "drop" in I he early magneto switch- 

 board it has come to be numbered li\ niillions and varies in type 

 from the simple electro-magnet wliirii operates a single contact to 

 the vacuum tube and the complete struiUire which cfTi-cts an entire 

 series of switching operations. 



When a small number of stations is inxoKed in a communicating 

 system complete fle-\ibilit\- of connection may be obtained by means 

 of simple relays controlling a small nimiber of contacts. As the 

 number of stations increases the nimiber of .switching operations 

 becomes so great that the use of simple rela\s which control small 

 numbers of contacts is not econoinical. The use of power driven 

 selectors and sequence switches and electro-magnetically operated 

 switches for completing a series of switching operations has therefore 

 become necessan,-. 



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