180 



BELL SYSTEM TECHNICAL JOURNAL 



Turning now to the effect of contact force on contact resistance: 

 we see (Fig. 16) that large and approximately reversible resistance 

 changes are produced as the force is varied repeatedly between fixed 

 limits. This shows that the effect is in the main elastic, though the 



320 



300 

 280 



260 



240 



220 

 <n 

 a. 200 



5 



< 180 



t 160 



a. 140 



D 



(J 



120 



100 

 60 



60 



40 



12 3 4 5 6 7 



FORCE IN DYNES 



Fig. 16 — Typical current-force cycle obtained with a single contact. 



existence of a narrow loop indicates a small plastic or irreversible 

 movement as a secondary effect. 



We have^ thus established that the current is conducted through! 

 solid carbon and that the deformations are mainly elastic. These! 

 facts give strong support to the "elastic theory" of "loose contacts," 

 i.e., the hypothesis that the change of resistance takes place becausel 

 of a change in contact area under pressure. An extensive study off 

 the resistance-force characteristics gave results which could not be 



