94 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



combustion engines) or the pulse time or the pulse frequency are 

 measured. 



In the capacitive tachometer a capacitor is alternately charged 

 from a voltage source and discharged through a meter, as shown in 



-±E ±C 



Fig. (1-3)6. Circuit furnishing an output pulse 

 of a duration inversely proportional to the 

 velocity of a bullet traveling the distance A.s 

 and opening, in succession, the switches A 

 and B. 



Fig. (1-3)7. Capacitive tachom- 

 eter; the output current is 

 proportional to the rotational 

 velocity of the contact arm A 

 but independent of the direction 

 of rotation. 



Fig. (1-3)8. Capacitive tachometer 

 with direction-sensitive indication. 



Fig. (1-3)7. The average discharge current / is proportional to the 

 rate of operation, n. 



I =CEn 



Another form of such a capacitive tachometer is illustrated in Fig. 

 (1-3)8. This system, in contrast to that shown in Fig. (1-3)7, fur- 

 nishes an output current which changes direction if the sense of rota- 

 tion is inverted. 



For general references on tachometers, see the book by Pflier, op. cit., p. 244; 

 also Ivan A. Greenwood, Jr., J. Vance Holdam, Jr., and Duncan MacRae, Jr., 

 "Electronic Instruments," in M.I.T. Radiation Laboratory Series, vol. 21, sec. 

 4.5, and John F. Blackburn, "Components Handbook," in M.I.T. Radiation 

 Laboratory Series, vol. 17, pp. 372ff, both McGraw-Hill Book Company, Inc., 

 New York, 1948. 



