92 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



the displacement can be converted into an electric signal with one of 

 the displacement transducers described above. Such a signal can be 

 converted into a velocity-proportional signal by the use of an inte- 

 grating network, as shown in Fig. (1-3)2. In general, systems in- 

 volving integrating networks are preferable to those illustrated in 

 Fig. (1-3)1, since differentiating networks tend to increase the noise 

 level. 



b. Induction Transducers. The only transducer which converts 

 the velocity of a movement directly into an electric signal is the 



Fig. (1-3)3. Tachometer for 

 d-c output with fixed magnet 

 and movable armature. 



Coils 



Fig. (1-3)4. Tachometer 

 for a-c output with fixed 

 armature and moving 

 magnet. 



Fig. (1-3)5. Tachometer for 

 a-c output with fixed magnet 

 and coils and moving yoke. 



induction transducer described in 1-22/. The system is frequently 

 used for the measurement of angular velocity, i.e., as a tachometer. 

 Three different forms of such tachometers are commonly available. 

 The first, shown in Fig. (1-3)3, is a d-c generator usually built with 

 stationary magnet and rotating armature; the output from the 

 armature is led to a collector. The magnitude of the output voltage 

 is generally of the order of 10 mV/rpm. The system permits the use 

 of d-c instruments for the measurement of speed and direction. In 

 some applications, the collector noise and ripple voltages are likely 

 to cause difficulties. The second system is shown in Fig. (1-3)4; it 

 consists of a rotating magnet and a stationary coil and furnishes an 

 a-c output. A third form, also for a-c output, is illustrated in Fig. 

 (1-3)5; it has both a stationary magnet and a stationary coil but 



