REi..iys IX iiir. nr.i.i. vis// u 9 



reluN' lo loiitrol a cliMriiii; nut (Imp. I'liis nimlilu-d ii-lonr.ipli rrla\- 

 is shi)\vn in Fig. 3 and is of particular inlerest as representing the 

 first step in the tle\eli)pnient of tiie telei)li()iie rela\-. 



In the magneto systems the indicator or drop was of the first im- 

 portance, so its develoi)meiU was rapiil. It was finalK- arranged in 

 one extensi\el\- usi'd >\-slcni witli two coils, wliicli were known as 



Fij;. 3 — Early telegraph relay used as telephone drop 



the line coil and the restoring coil. The magneto current from the 

 subscriber's station energized the line coil to drop a shutter which 

 was restored through the agency of the restoring coil when the operator 

 inserted a plug in the associated jack. The early development of 

 the drop undotiblcdK- influenced the forms of relay structures which 

 were developed a little later. The analogy between the line and 

 restoring coils of the magneto system and the line and cutofif relays 

 of the common battery system is very close. In the latter the cur- 

 rent over the subscriber's loop energizes the Hne relay which lights 

 the line lamp. The insertion of the plug in the subscriber's jack 

 energizes the cutoff relay which opens the circuit through the line 

 relay and thus extinguishes the light. In addition, the line and cutoff 

 relays are assembled on a common mounting plate, forming an ap- 

 paratus unit, although they are not parts of the same structure as 

 were the corresponding coils of the drop. 



The early telephone relays resembled more closeK" in construc- 

 tion and form the early drop than they did the telegraph rela>-, 

 although the influence of design work on the telegraph relay appears 

 in the development of later types of telephone rela\s. The earl\ 



