Design Characteristics of Electromagnets 

 for Telephone Relays 



By D. D. MILLER 



Note: The electromagnets described are confined to relays, although 

 the principles involved apply as well to selector magnets, clutch magnets 

 and electromagnets in general. A treatment from the viewpoint of the 

 telephone engineer is given of the important considerations which deter- 

 mine the design of the magnetic parts of relays and the economics of the 

 winding dimensions. A knowledge of these factors as well as of the general 

 considerations which are discussed is of great importance in the selection 

 and application of relays to the telephone system. The operating and 

 economic importance to the Bell System of the great number of relays 

 required in the operation of the plant has been described in a previous paper.' 



lNTR()in( HON 



ELECTROMAGNETS or rela>s its generally used in telephone 

 switchboards are simply switches which are controlled electro- 

 magnetically. These switches may be required to open or close a 

 number of separate and distinct circuits simultaneously or in a certain 

 sequence. In many cases it is essential that the relay switch be 

 opened or closed very quickly as this time may have a direct influence 

 on the amount of apparatus required and consequently the first 

 cost of the plant. The operating time of the relays also has a direct 

 influence on the time retiuired to establish a telephone connection. 

 The above statements are particularly evident in automatic systems 

 where selector apparatus is required to establish a connection between 

 parties but is released during the conversation. It follows that the 

 number of selector circuits and relays therein depends upon the 

 amount of trafific and time required for the selectors to establish the 

 connection. 



To establish a telephone connection between two parlies in certain 

 automatic telephone systems, requires the opening and closing of 

 about 2,000 electric .switches of which 1,200 are operated by simpler 

 types of electromagnetic relays. In a typical manually operated 

 system a call is completed by liu- ()|)eiiing and closing of about 112 

 switches of which 70 are operated b>- rela>s. It is therefore evident 

 that the relay switches must operate both quickly and reliably and 

 maintain a high degree of stability throughout a long period of scr\ice. 



In controlling the various circuits in telephone systems by relays, 

 the character of the circuits determines the construction of the rC'lay 

 switches. If large currents are to be controlled the relay switch 



' Relays in the Bell Svslem, S. I'. Sl,ackK-lt)ii .m.l II. W. I'mcrll, Hell .Svslem Tech. 

 Journ., Vol. 3, p. 1, iy24. 



206 



