70 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



i.e., the sensitivity S' decreases as the overlapping area increases (for 

 a fixed width; the width has no influence upon the sensitivity). The 

 characteristic AC/C versus Aljl is linear. 



Figure (1-2)54 illustrates two capacitive transducers for rotary 

 motion, a single and a differential transducer. The sensitivities and 

 the transfer characteristics are analogous to those for linear move- 

 ment. The characteristic C =f(<p) can be modified by appropriate 



(a) (b) 



Fig. (1-2)54. Capacitive transducers for rotary motion: 

 (a) two-plate capacitor; (b) differential capacitor. 



shaping of one of the two plates. The sensitivity of either type (also 



that for longitudinal-motion transducers) can be increased by 



multiple-plate arrangements. 



Another method to obtain high sensitivity is by the use of the 



serrated-type transducer shown in cross section in Fig. (1-2)55. A 



longitudinal shift of one capac- 

 itance plate by an amount x against 

 the other has the effect of changing 

 the capacitance from the minimum 

 to the maximum value. The trans- 

 fer characteristic is similar to that 

 in Fig. (1-2)53. A sensitivity of 

 the order of l/^F/0.0001 in. has 

 been obtained. 



y-M 



-B 



Fig. (1-2)55. Serrated-capacitor dis- 

 placement transducer. 



Capacitive transducers have been used for the measurement of 

 extremely small displacements down to about one-tenth of an ang- 

 strom unit(10 -9 cm, 4 x 10 -10 in.), i.e., about one-tenth the diameter 

 of a hydrogen atom. 1 On the other hand, they have been proposed 

 for the measurement of distances between 3 and 30 m in airplane 

 altimeters. 2 The change-of-distance method is generally preferable 

 for the small and very large ranges; the change-of-area method is 

 used for displacements of the order of centimeters. 



1 Natl. Bur. Standards News Bull., 42 (1), 1 (1958). 



2 P. C. Sandretto, Proc. IRE, 32, 167 (1944). 



