1040 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1952 



techniques. The basic requirements for satisfactory welds are: 



1. Sufficient strength to withstand the forces encountered during 

 manufacture and service; 



2. Accurate positioning of the contacts on the wires, and 



3. Low cost. 



These requirements apply to both stationary and twin contacts. How- 

 ever, because of differences in geometry, entirely different methods have 

 been developed for welding the two types of contacts. 



The twin contacts are produced by spot welding precious metal con- 

 tact tapes to the tips of the twin wires. The diagram of the welding 

 circuit is shown in Fig. 13. The condenser c is charged by a power supply 

 to a predetermined voltage. The condenser is then discharged through 

 the primary of the welding transformer t giving rise to the low voltage 



r,^,.,^r, ELECTRONIC 

 POWER SWITCH 



SUPPLY 



TWIN WIRE 

 'ASSEMBLY 



Fig. 13 — Diagram showing the essential elements of the spot welding process 

 used for the twin contacts. 



liigh current surge which produces the weld. The contacts are then 

 sheared to length and the surfaces are coined to a C3dindrical shape. 

 The spot wielding process did not appear best for welding the station- 

 ary contacts to the ends of the wires because of the need to grip the wires 

 with heavy welding electrodes in the limited space directly behind the 

 contacts. Accordingly, a type of welding known as "percussive welding" 

 was developed, which permits one of the electrodes to be placed near 

 the wiring end of the wire springs without developing excessive heat 

 in the wires and which also permits the accurate positioning needed for 

 the contacts in order to control the point of contact closure on the as- 

 sembled relay. The welding circuit is shown in Fig. 14. The condenser c 

 is charged by means of a direct current power supply, and the condenser 

 voltage also appears on the stationary wire. As the contact to be welded 

 is moved toward the end of the wire, the condenser discharges forming 

 an arc which melts the abutting surfaces of the contact and wire. The 

 constants of the electrical circuit and mechanical system were chosen to 

 assure melting a proper amount of metal at a controlled rate to assure 

 high weld strength. The parts are held together during the very brief 



