10 BELL SYSTEM TECIIXICAL JOVRXAL 



telcphune relay shown in I'ii;. 4 lias liule relation with the modified 

 telegraph relay of Fig. 3. It is much smaller and lighter than the 

 first relay and, in addition, there is a distinctiv-e structural change 

 in that the armature is suspended by a reed hinge. 



At this time the limiting conditions controlling the operation of 

 either telephone circuits or the apparatus in them had not been 



Fig. 4 — Early telephone drop relay 



established with much precision, so that the requirement for a relay 

 was, roughly, that it should do the work required and any arrange- 

 ment more sensitive and reliable than a previous arrangement was 

 an improvement. The principle of the reed hinge for an armature 

 support was sound, in that it pro\ided for a good magnetic circuit 

 and an easy means for close air gap adjustment and it is now used 

 extensively with rcla\s of the latest design. 



Line, Cutoff and Supervisory Rel.\vs 



When the common battery system was dev^eloped, however, it 

 was found that the reed hinge relay was not capable of meeting the 

 additional requirements imposed by the new system. The common 

 battery cord circuit originally suggested is shown in Fig. 5. It is 

 apparent that the relay shown in the diagram must indicate positively 

 to the operator the position of the switch hook in the substation set 

 and must respond to the motion of the switch hook if the subscriber 

 moves the hook up and down to interrupt the circuit. In addition, 

 as this relay is in the transmission circuit it must not introduce 

 objectionable transmission losses. The reed hinge relay could not 

 meet these additional requirements, and accordingly a new relay 

 was designed especially for the common battery system that was 

 the most important single factor in making the new system possible. 

 In order to obtain an armature that would respond quirkK- to an\- 

 change in the holding magnetic force all forms of sup|)ort for the 

 armature were rejected. The relay developed is shown in Fig. ti 



