532 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



WHAT IS A GOOD CONNECTION? 



The quality of a connection depends fundamentally on two factors: 

 the contact area and the contact pressure. As long as there is sufficient 

 pressure and the atmosphere cannot enter the joint, the connection is 

 considered a good one. If, however, the elastic energy which holds the 

 two surfaces together is small, various disturbances may cause a partial 

 separation of the interlocking metal particles and thus effect a change 

 in resistance. For normal telephone applications a good connection may, 

 therefore, be defined as one which not only has sufficient contact area 

 and contact pressure but which also has sufficient elastic reserve to 

 maintain contact area and contact pressure throughout the desired life, 

 which may be forty years or more. 



The mechanical disturbances to which a connection may be subjected 

 are: handling, vibration, temperature changes and cold flow. 



HANDLING AND VIBRATION 



The solderless wrapped connection is well protected from the point 

 of view of handling and vibration. The locking effect on the rectangular 

 terminal or a terminal having well defined edges does not permit loosen- 

 ing of the center turns from the terminal. In vibration tests where con- 

 ventional soldered connections were compared with solderless wrapped 

 connections, it was found that solderless wrapped connections outlast 

 soldered connections. This is due to the fact that a sudden change in 

 cross section from wire to solder lump localizes the stresses at a very 

 small area. (See Figs. 7 and 8.) In the screw connection a similar con- 

 dition exists where the wire emerges from under the screw head. In the 



CLIPPING 



0.125' 



Fig. 7 — Standard solder connec- 

 tion of U-relay. 



TAPERED STIFFNESS 



Fig. 8 — Solderless wrapped con- 

 nection of modified U-relay terminal. 



