988 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952 



when the relay is operated. As will be noticed, the further deflection of 

 the contact spring necessary to obtain the required contact force after 

 closure, causes the moving contact to slide and rock slightly on the fixed 

 contact. For the reasons previously mentioned such a contact is prone 

 to lock when the conditions are favorable. 



Now, the bottom of the figure shows a card actuated contact spring 

 arrangement. Here a phenol fibre card is employed to operate the contact 

 instead of a stud. The moving contact spring itself is pretensioned against 

 the fixed contact to give the desired contact force. The card is held by 

 two card springs that are tensioned away from the fixed contact in a 

 slightly greater amount than the moving contact spring is tensioned 

 toward it. As a result, when the relay is unoperated, the card holds the 

 make contact away from the fixed contact. When the relay operates, the 

 armature pressing against the top of the card pushes it toward the fixed 

 contact and allows the contact to close. It is quite apparent that with 

 this actuation the moving contact engages the fixed contact without 



STUD 

 ACTUATED 



CARD 

 ACTUATED 



SPOOL-HEAD SPRING 



^ 



UNOPERATED 



OPERATED 



UNOPERATED 



Fig. 6^Two methods of contact spring actuation and their influence on contact 

 locking. 



