1388 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1956 



Fig. 3 — ■ Micro-photograph showing successive stages in the formation of the 

 contact spring. The posts are 3*0 inch in diameter. 



(a) 



(b) 



Fig. 4 — Cathode-raj^ oscilloscope display of wafer unit static characteristic: 

 (a) before and (b) after tapping. 



the "S" configuration in a forming jig. By an electrolytic process the 

 spring is then cut to the proper length and pointed. The niicro-photo- 

 graphs in Fig. 3 show successive stages in the formation of the contact 

 spring. 



In the final assembly of the unit the nickel rod with the contact spring 

 is pressed into place until contact is made with the silicon. It is then 

 advanced a half mil to obtain the proper contact pressure. The voltage- 

 current characteristics as viewed at 60 cycles on a cathode-ray oscillo- 

 scope will then appear as shown in Fig. 4(a). The unit is "tapped" into 

 final adjustment. This is done by clamping the unit in a holder and 

 rapping it sharply on the top of a hard wood bench. This procedure re- 

 quires experience as excessive "tapping" will impair the performance 

 of the unit. Usually one vigorous "tap" is sufficient to produce the 

 desired effect and the voltage-current characteristic will appear as 



