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of the switchboard type, relays, jacks, and interrupters. Certain 

 apparatus known as switches employ these springs as brushes or 

 wiping members. Here precious-metal contacts are not usually 

 employed, but the ends of the springs must serve as contacting and 

 wearing parts. Other springs are employed statically or, in other 

 words, are required to maintain constant pressure for long periods 

 without interruption. All springs that are attached to, or form part 

 of, electrical circuits are soldered to connecting wires. This is usually 

 done by soldering the wire or connection to a lug or projection that 

 forms part of the spring. These lugs in most instances are on 

 the opposite end of the clamped area from the operated end. The 

 springs are almost always clamped between strips of phenol fiber 

 because of its good insulating properties, mechanical strength, and 

 permanency of form. A typical example of the use of the more 

 common type of these springs is illustrated by Fig. 1 showing the 

 familiar switchboard key. 



Figl — Common type of switchboard key illustrating use of sheet-metal springs: 

 A, plunger spring with crimp; B, crook spring; C, straight plunger spring. 



The properties required of these small springs which are numerous 

 and vary for each type of application are summarized as follows: 



The proportional limit must not be too high to prevent the spring's 

 being adjusted by flexing it with a tool to the point where it takes a 

 set and occupies a position where it provides the desired operating 



