SOLDERLESS WRAPPED CONNECTIONS 



PART II 



577 



2. The amount of recovery from strains of 20 to 50 per cent should 

 not be greater than 20 per cent. 



PERMANENCE OF WRAPPED SOLDERLESS CONNECTIONS 



By the photoelastic, photoplastic and strain analysis of the previous 

 two sections, it has been demonstrated that the wrapped solderless 

 connection is held together by the hoop stress in the outside wire whose 

 value is determined by the winding stress and the locking in effect 

 dependent on a dissymmetry of the terminal. The high stresses cause 

 plastic flow in the wire and terminal in such a manner that the two 

 materials flow together and produce an intimate air tight joint. The inti- 

 mate nature^ of this contact has been demonstrated by dip coating nickel 

 silver terminals with pure tin and wrapping them with cleaned bare 

 copper wire. The wrapped terminals were then placed in a glass tube, 

 evacuated, sealed off and heated for 400 hours to 180°C (37°C below 

 the melting point of tin). The samples were then removed, mounted 

 vertically, polished, etched and examined microscopically for distinguish- 

 ing constituents. It is believed that if such a constituent appeared on 

 the originally bare copper wire after such treatment, that the contact 

 was sufficiently intimate to permit solid state diffusion. A section of the 

 wire in contact with the corner is shown by Fig. 19. The copper is seen 



COPPER 



NICKEL 

 BRASS 



Fig. 19 — Solid state diffusion of tin into copper in a wrapped solderless con- 

 nection. 



' This experiment was conducted by G. S. Phipps. 



