33 

 TRANSDUCERS 



K. E. MACHIN 



INTRODUCTION 



The use of electronic instruments is not confined to purely electrical measure- 

 ments. By means of suitable devices mechanical displacements, light inten- 

 sities, temperatures and other physical quantities may be converted into 

 electrical signals. When these signals have been amplified or modified by 

 electronic circuits they are usually re-converted to displacements, either of a 

 meter pointer or of a cathode ray tube spot. All the devices which convert 

 quantities of one type into equivalent quantities of another type are known 

 as transducers; thus a mechanoelectrical transducer produces an electrical 

 output corresponding to a mechanical input, while an electroacoustical 

 transducer converts an electrical signal into the equivalent sound. 



Thermoelectrical and opticoelectrical transducers are discussed in Chapters 

 29 and 28 under their more familiar names of electrical thermometers and 

 photocells; the cathode ray tube, yet another example of a transducer, is 

 described in Chapter 32. The present chapter deals only with mechano- 

 electrical (m-e) and electromechanical (e-m) transducers, and with their 

 application to biological research. 



Electronic instrumentation inevitably involves expense and complication, 

 and it is therefore important to consider under what circumstances its use 

 is preferable to visual or mechanical methods of achieving the same end. 

 Is it better, for example, to measure the contraction of a muscle with a lever, 

 a stylus and a rotating smoked drum, or with a mechanoelectrical transducer, 

 an amplifier, a cathode ray oscillograph and a moving film camera ? 



Electrical transducer techniques are particularly valuable when the mecha- 

 nical quantities to be measured either vary very rapidly or are very small. 

 A mechanical recording system inevitably possesses considerable inertia, and 

 it will therefore be unable to follow rapid variations of input. A transducer, 

 on the other hand, can be very small and light, and be made to reproduce 

 faithfully practically any movement which may originate from a biological 

 preparation. When very small mechanical quantities are to be measured, 

 refined engineering techniques are needed to produce the necessary mechani- 

 cal amplification. Electrical amplification of the signals from a transducer, 

 on the other hand, can be provided straightforwardly and to an almost 

 unlimited extent. Furthermore, a mechanical recording system, particularly 

 one of large ampHfication, will produce considerable 'loading' of any 

 preparation to which it is attached; this effect can be made very small by 

 using instead a suitable transducer, and moreover is quite independent of the 

 amount of subsequent amplification. 



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