540 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



40 



60 



80 100 



TIME IN MINUTES 



Fig. 17 — Relaxation of wrapped wire after heating for three hours at 175°C. 

 The reserve twist of 14° is an indication of the tension left in the wire which 

 amounts to 47 per cent of the original wrapped tension. 



A severe test for a wrapped connection is heating to 175°C for three 

 hours. This relaxes, about half the stress and is considered the equiva- 

 lent of a 40-year life at 135°F. Position 5 in Fig. 12 is the same as Posi- 

 tion 3, that is, the wire has just been wrapped onto the terminal and 

 its tension is 1210 grams. If this connection is now heated to 175°C and 

 the angle of twist noted every fifteen minutes, a curve is obtained as 

 shown in Fig. 17. It should be noted that at 105 minutes the curve is 

 for all practical purposes asymptotic at an angle of 23°. If the heated 

 connection is cooled and unwrapped and the set in the terminal meas- 

 ured, it is found to be 9°. The 14° difference is a measure of the elastic 

 reserve. This is 47 per cent of the wire tension before heating. The cor- 

 responding tension WF is then 570 grams. This process is illustrated in 

 positions 6 and 7 of Fig. 12. Position 7 shows that the terminal set of 

 9° was the same as before heating. 



A similar experiment was made with formex insulated wire wrapped 

 on a nickel silver terminal. Instead of subjecting the connection to the 

 heat of an oven, a high current was passed through the wire. Essentially 

 the same curve as shown in Fig. 17 was obtained. 



To further check the behavior of springs with complex elastic defor- 

 mation such as in a wrapped connection on a terminal having edges, 

 measurements were also made with simple helical springs tensioned 



