MEASURING FORCES A\D Wl'.AU I\ SWITCHING APPARATTS 499 



lIcMice in agrconiont witli the dahi of l''i<;. I."), the (lisplaccnicnt I'oi- no 

 gross slide should viwy as I lie Iwo-thirds power of llic normal force. 



Anothei- deduction from Iviuatioii (2'2) is that the displacement for 

 no slide should vary as the iiiNcrse two-thirds power of the shear stiff- 

 ness constant pi. For example <;um inibher with ti shear stiffness of 2 x 10 

 dvnes cm' should give 100 times the displacement of a ])lastic with a 

 shear stiffness of 2 x lO'" dynes em". A rough check of this deduction 

 has l)een made l)y cementing a thin sti'ip of gum rubber on the face of 

 a shear responding ceramic and with a normal force of 'M) grams (O.OOiio 

 pounds), vibi-ating the wire at its full amplitude of 2 mil inches. Over this 

 range the voltage response was sinusoidal indicating that no gross slide 

 took place. This is 33 times as large a motion as occurred for a plastic 

 with an elastic stiffness 1,000 times that of the rubber and verifies the 

 \ariation of 8x with ju. 



The other experimental (luantity that can be obtained from Equation 

 (22) is the radius /• of the effective contact points of the plastic. If all 



400 

 300 



O 30 



10-3 



DOUBLE DISPLACEMENT IN INCHES 



Fig. 23 — Plot of area of force displacement loop against double displacement. 



