SOLDERLESS WRAPPED CONNECTIONS — PART I 527 



of the surfaces touch and large currents passing through such a connec- 

 tion may develop heat and melt the metal at the high spots. 



CONTACT FORCE 



To make the above mentioned area of one square centimeter effective 

 for electrical conduction it is necessary to press the two metal parts 

 together with a force so high that essentially all particles of the area are 

 intimately interlocked and free from insulating impurities. If the pres- 

 sure is high enough, the film which appears in the form of oxide on the 

 terminal surface is crushed. In general it is assumed that in a good con- 

 nection the contact force should be such that the contact area produced 

 is equal to or greater than the cross sectional area of the wire. In screw 

 connections, crimped connections and wrapped connections the contact 

 area is normally a multiple of the wire cross sectional area. In plug con- 

 nections, such as on vacuum tube sockets, the contact area is very small. 

 In a Fahnestock clip for example, the contact area is about one quarter 

 of the cross sectional area of the wire. 



LIFE 



If the electrical resistance of a pressure joint is to remain constant 

 with time, it is the contact area which must remain substantially con- 

 stant, but not necessarily the contact force. Once the metal particles are 

 tightly interlocked a subsequent reduction in contact force within rela- 

 tively wide limits does not change the electrical resistance. The resist- 

 ance will increase only when the force is reduced to such a low value 

 that vibration and handling cause partial separation of the contact area. 

 In such a case two changes may take place : 



1. The atmosphere may enter through the fringe of the contact area 

 and a process of corrosion may begin. 



2. The effective contact area may be reduced through dislodging 

 some of the contacting particles. 



In both cases the resistance is increased. Therefore, to produce a 

 durable connection it is important to have a firm joint and one such that 

 the atmosphere cannot enter the contact area. The term commonly 

 used for such a joint is ''gas tight." 



ELASTIC RESERVE 



The question now arises how much reduction in contact force can be 

 tolerated before a joint loses its gas tightness? In all types of pressure 



