RELAYS AND RELATED MECHANISMS 



of diagrams similar to Figure 34.1 is then both unnecessary and unfor- 

 givable. 

 The symbols used in relay circuits are summarized in Figure 34.3. Contacts 



Relay coils Relay contacts 



500 A/3 Plain 



Twin coil 



Polarized 



A2 Make 



Break 



(M) 



(B) 



Change-over (C) 



^ 



B 



Slow release 



Slow operate 



High speed 



(Figures in box give resistance in ohms. 

 Letter is relay designation. 

 Figure after oblique stroke gives 

 number of contacts.) 



Make -before- (K) 

 break 



(Letter is relay designation. 

 Figure is serial number of 

 contact set.) 



Uniselectors 



Non-bridging 

 wiper and 

 bank contacts 



Figure 34.3 Symbols used in relay circuits 



Same, but 

 bridging 



Driving 

 magnet 



Interrupter 

 contacts 



are always drawn in the unoperated position; thus a 'make' contact is 

 drawn open, and a 'break' contact closed. In a complicated circuit the 

 symbols for 'earth' and 'earthed battery' are used wherever a connection 

 to one or other pole of the supply occurs (see Figure 34.2); it is of course 

 not thereby implied that as many batteries as symbols are provided. 



THE POST OFFICE 3000 TYPE RELAY 



General construction 



The components of a 3000 type relay are illustrated in Figure 34.4. A soft 

 iron yoke surrounds a detachable coil wound on an iron core. An L shaped 

 armature rests on a knife-edge machined on the yoke, and is retained by 



512 



