64 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



of a permanent bar magnet, one pole extending inside of a coil with 

 many turns. An axial relative movement between the magnet and 

 the coil causes an output voltage. 



The use of Alnico V magnet is recommended because the poles in 

 such a magnet are short and well defined. The sensitivity is 



S = - = -w'O X 10- 8 



v 



where e is the output voltage in volts, v the velocity in centimeters 

 per second, n' the number of turns per centimeter of coil length, and 



Circular 

 gop { 



Central 

 core 



Fig. (1-2)47. Induction-type dis- 

 placement transducer for rotary 



movement. 



Coil "-Permanent 

 magnet 



Fig. (1-2)48. Induc- 

 tion-type displacement 

 transducer for longi- 

 tudinal movement. 



O the flux in gauss per square centimeter through a cylindrical sur- 

 face surrounding one-half the magnet. Practically obtained sensi- 

 tivities are about 0.01 volt/(cm/sec). 



Figures (1-2)47 and (1-2)48 show two forms of moving-coil trans- 

 ducers. The system illustrated in Fig. (1-2)47 is similar to the usual 

 meter movement and can be used for the conversion of rotary move- 

 ments into electric signals; that illustrated in Fig. (1-2)48 is similar 

 to the movements used in moving coil or electrodynamic speakers 

 and can be used as a transducer for longitudinal movement. The 

 output voltage is 



e — Ba?iv cos (p ( 1 ) 



where B is the flux density in the air gap, a the coil area, n the number 

 of turns, v the velocity with which the coil moves, and cp the angle 

 between the coil plane and the magnetic-field lines. The angle <p in 

 Eq. (1) is usually negligible; in the system in Fig. (1-2)47 the angle cp 

 is zero in the position illustrated, and the angular displacement is 

 small so that cos cp differs from one by an insignificant amount. In 

 the moving-coil system with circular pole pieces and in the system 

 shown in Fig. (1-2)48 the fields extend radially and <p is always zero. 

 Moving-coil systems of the type shown in these figures have been 



