OTHER DESIGNS OF RELAY 



to avoid too violent a mechanical shock to the tube as the relay operates. 

 A resilient mounting also permits slight expansion of the glass as the switch 

 warms up under load. 



If switch operation must be positive and free from 'bounce', some elastic 

 connection between the relay armature and the switch is also desirable to 

 prevent splashing of the mercury. 



Mercury switches are also obtainable with the relay armature enclosed 

 within the glass. An external electromagnet is then needed to move the 

 armature and hence produce contact operation. A plunger switch of this 

 type mounted within a solenoid can control currents up to 60 amps. 



All mercury relays are orientation-sensitive, and it is essential that they be 

 fixed at the correct angle on a rigid mounting. 



For further details of the types of switch available, manufacturers' litera- 

 ture should be consulted. 



Thermal relays 



Electromagnetic action is not the only method of achieving mechanical 

 movement in a relay: expansion caused by electrical heating is the basis of 

 the operation of thermal relays. 



In the thermal delay relay {Figure 34.16) the operating current is passed 

 through a heating resistance wound on a bimetallic strip; when hot the 



Bimetal ^ . ^^ 



strip Heater ^^iT 



[^J ^- Contacts 



Lever 



■J" j"j"j'j ^^^ 



Break 



Figure 34.16 Thermal delay relay shown diagrammatically 



Spring—- 



Contacts 



Evacuated 

 container 



I Pivoted 



"armature 



Insulated 

 bobbin 



Multi-turn 

 hot-wire 

 winding 



Figure 34.17 Hot-wire vacuum switch shown diagrammatically 



523 



