MEASURING FORCES AND WEAR IX SAVIT(^HIXG APPARATITS 



475 



previoius paper," this value can be held veiy eoiistant and can be con- 

 trolled by controlling the bias on the limiting device. The magnitude of 

 tlie force can be determined by comparing the voltage with tiuit ob- 

 tained l)y suddenly lifting a weight off liie ceramic and has been adjusted 

 to e([ual 500 grams. The voltages shown then correspond to the open 

 circuit voltages generatcMl b>- api)Iying 500 grams to a point at the center 

 of the ceramic. 



As shown in the appendix, tiie effect of applying a force at a point in 

 a ceramic is not the same as that caused by distril)uting the force uni- 

 formly over the surface due to the fact that radial strains are generated 

 and the.se act through the radial piezoelectric constant to reduce the 

 \'alue generated })y the thickness piezoelectric constant. It is shown that 

 I he point application of stress generates only 40 per cent as much as would 



BARIUM TITANATE 

 (CLAMPED) 



CATHODE 

 RAY 



OSCILLATOR 



4//F 



Fig. 3 — Circuit used to measure open circuit voltages for barium titanate 

 samples. 



