8 BELL SYSTEM TECHSICAL JOVRSAL 



nifiit and cosi of opi-r.itioii and maintenance. The nmre conipli- 

 catcd cirniit or llie one neeessitaling additional efiiiipnient ina\- Ik 

 sufticicnty more relial)le to justify its use. 



In consiclerinjj the application of relays in any telephone circuit, 

 a gi\en problem is iisiialh' presented and the various possible methods 

 of accomi)lishing the desired end are considered. These nicthods 

 may involve combinations of relays or of relay parts which <l(i not 

 exist and may even involve combinations which arc entirely imprac- 

 tical or uneconomical of application. Any simplifications which ma\ 

 be elTected are considered and in case the design of any apparati:s 

 may effect appreciable savings in the circuit or otherwise appear 

 justifiable, this nia>' be undertaken. Such requirements on rela\' 

 design are, of course, subordinated to any general design cf>nsider- 

 ations, such as relay structure, etc., which may be governing from the 

 standpoint of the economic production of the relays themselves. 



A few considerations which influence the selection of relavs and 

 which are very closely associated with the fundamental relav- design 

 may be considered from the standpoint of their efTect on telephone 

 circuits and their application in the field. Ii would, of course, be 

 impossible within the scope of this paper to describe all the relay ap- 

 plications in modern telephone practice, but a discussion of the relax 

 reciuirements for a few typical cases will serve to illustrate the ijrin- 

 ciples invoked. While the first relays used in the telephone svsteni 

 were telegraph relays adapted for use in signaling, the vast majorii\ 

 of rclavs now in use in the Bell S\stem are designed primarily- for 

 telephone circuits. The requirements are usually quite different, 

 particularly as regards the energy available for operation, the speed 

 of operation and reliabilit\- of coiitacls and in most cases the cost. 



Early Tki.i:piu)ni-: Rix.ws 



In the first telephone switchboard for commercial service which 

 was installed in 1S7S, the electro-magnetic devices consisted of a 

 telegrajih relay and an annunci.ilor for each subscriber's line, ,iiid a 

 call bell common to .ill liiu>. Of these three the telegniph rel.ix was 

 the largest and most costly, so the tlesirability of reducing the num- 

 ber required and of providing a smaller anfl cheaper apparatus unit 

 was ajiparent. Changes -were soon made in the magneto svstem ih.ii 

 removed the relay from the subscriber's line and associated c.k h 

 relay with a group of lines for sui)ervisory purposes. In the early 

 switchboard, patented in 1S7!), from which the standard switchboard 

 was developed, ,i modified lelegr.q)h relav' aiijicued as a clearing out 



