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BELL SYSTEM TECHNICAL JOURNAL 



remove it. All of this protection is, of course, to shield the apparatus 

 from mechanical vibrations and acoustic disturbances. 



Fig. 19— Exterior view of complete experimental arrangement. 



With this apparatus we investigated the variations in displacement 

 and resistance when the forces are varied cyclically between fixed 

 limits. Measurements on a large number of contacts are summarized 

 in curves. Figs. 20 and 21. The cyclic characteristics, though some- 

 what irregular and having the form of narrow loops, approximate 

 straight lines when the variables are plotted on logarithmic scales. 

 Only one complete characteristic is shown in each set of curves, other 

 typical measurements being represented by dotted straight lines 

 joining the end points of their respective cycles. The full line in 

 each figure represents the cycle of a typical contact, obtained by 

 averaging, over the range in which the difTerence between the maximum 

 and minimum force limits or maximum and minimum displacement 

 limits is relatively large, in which case the slope is apparently con- 

 stant. 



If we let N" and N represent the slopes of the typical force-displace- 



