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THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1952 



II. METHODS FOR MEASURING NORMAL AND TANGENTIAL FORCES 



In order to investigate the performance of a mechanical device and 

 the causes of wear in it, it is desirable to be able to measure the forces 

 occurring in various parts of the device. To measure the complete per- 

 formance it is necessary to measure not only the slowly applied forces 

 but also the very short time dynamic forces that occur when various 

 parts of the device impinge on each other. 



The most common method for measuring such forces is by means of 

 a piezoelectric crystal such as quartz. Quartz, however, has the disad- 

 vantage that it is not very sensitive and also that it has such a low di- 

 electric constant that the input impedance of any amplifier associated 

 with it has to be prohibitively high if forces varying as slowly as a one- 

 tenth of a second are to be measured. Since the impedance of the oscil- 

 lograph or amplifier is usually lower than that of the crystal, the crystal 

 having the greatest sensitivity will be the one which generates the most 

 charge for a given force, which corresponds to the crystal ha\'ing the 

 largest d piezoelectric constant. Table I shows a tabulation of the d 

 constants for compression and shear for several of the most common 

 piezoelectric crystals and for the ceramic barium titanate. The dielectric 

 constants are also given. 



Of these materials the only ones that have sufficient mechanical 

 strength to withstand the mechanical shocks they are subjected to in 

 the measurements of forces are quartz, tourmaline and barium titanate 

 ceramic. The crushing strength of the ceramic has been found to be 

 from 60,000 to 80,000 pounds per square inch. From the values of the 

 d piezoelectric constants it is seen that the barium titanate ceramic is 

 about 50 times as sensitive as quartz or tourmaline and it is possible to 

 use small pieces of the ceramic to work directly into cathode ray oscillo- 



Table I 



