TRANSDUCERS 



loudspeaker (both these values being for a current of 1 amp in 3 ohm coils). 

 With this force available, it is possible to use a very stiff spring across the 

 transducer to raise its output impedance ; movements of the order of 1 mm 

 are possible, the resonant frequency then being of the order of 200 c/s. 



'/y/y 



Armature 

 control 

 spring 



Figure 33.24 Moving-iron transducer with reasonable linearity 



Another configuration of this type of transducer is used in the moving-coil 

 meter (Figure 33.25). This is capable of only a very small mechanical 

 output, and in its usual form has a very low resonant frequency. An 

 ordinary moving-coil meter has however been adapted as an electrically 

 controlled torsion balance; other similar applications when a small force 

 only is required may suggest themselves. 



Pen 



Coupling 

 spring 



Coil 



control 



spring 



.Moving 

 coil 



(a) 



(b) 



Figure 33.26 Pen recorder: {a) moving-coil loudspeaker type ; {b) moving-coil 



meter type 



The moving-coil pen recorder (also known as the penwriter and pen 

 oscillograph) can be made using either the loudspeaker principle or the 

 meter principle; the two configurations are illustrated in Figure 33.26. 

 In each case the driving coil is controlled by a spring, and another spring 

 is used to couple the coil to the pen. Viscous damping by means of oil is 



494 



