TRANSDUCERS 



By the use of transducers and electronic instruments it is often possible to 

 present information in a form more convenient for subsequent analysis. 

 Thus the simultaneous variation of several quantities may be recorded by a 

 multi-channel pen recorder, even though the quantities may be as diverse as 

 pressure, acceleration, volume and voltage. Furthermore, it is possible by 

 suitable circuit techniques to perform mathematical operations before the 

 final recording ; if, for instance, information about the rate of change of a 

 mechanical quantity is required, it can be evaluated electronically and 

 recorded directly. Considerable time may thus be saved in the subsequent 

 record analysis. 



Finally, mention should be made of the possibilities of telemetry, whereby 

 measurements made with transducers on a freely moving animal can be 

 transmitted via a radio link to a remote recorder^'". 



Throughout this chapter it will be emphasized that the transducer must be 

 chosen carefully to suit the experiment. For this reason there can be no 

 single 'general-purpose' transducer, or even one class of transducer suitable 

 for all biological applications. Very few commercial transducers are available ; 

 fewer still are useful in biology. Inevitably, then, the biologist may be faced 

 with the necessity of constructing transducers to fit his own experiments. 

 Very few dimensioned drawings of transducers are given here; hardly 

 any photographs are used; instead, an attempt has been made to explain 

 the basic principles of the various types of transducer, and to illustrate some 

 typical designs in the hope that the reader will then be able to choose the 

 best type of transducer for his particular application, and, if necessary, design 

 and construct it himself. 



ANALYSIS OF THE MECHANICAL PROPERTIES OF 



TRANSDUCERS 



In all types of instrumentation it is essential to take into account the limita- 

 tions of the measuring equipment, and the distortions it may impose on the 

 measured quantity. Thus a lever attached to a biological preparation will 

 have a natural frequency of oscillation, and for faithful reproduction the 

 input quantity must vary slowly compared with the natural period. Further- 

 more, if a very rapid change of input is applied to such a lever the recorded 

 output will be a damped oscillation bearing no relationship whatsoever to 

 the input quantity. Although the mechanical components of a transducer 

 are usually small and light, they too will have a resonant frequency, which 

 sets a limit to the transducer's speed of response to a changing input. Since 

 transducers are particularly useful in dealing with quantities varying too 

 rapidly for mechanical recording this limit may still be a significant one. 



The formal technique for analysing the response of a mechanical system 

 to a given input involves the solution of the differential equation of the system. 

 However, electrical engineers, faced with the same problem in circuit analysis, 

 have evolved rapid geometrical and algebraic techniques for avoiding the 

 necessity for such a solution; in this section it will be shown that circuit 

 analysis techniques can be applied with but little modification to the analysis 

 of mechanical systems. Thus the theory and results of Chapter 5 can be used 



472 



