ELECTRICAL MEASUREMENTS 



179 



and these are relatively easy to measure. In other cases, the biological 

 process can lead to a change in resistance or capacitance or some other 

 property of a circuit. Movement can be detected by a mechanical coupl- 

 ing to an instrument which produces an electrical "signal." The modifi- 

 cations, interconversions, and variations are almost unlimited. 



Signal 



Input 

 transducer 



Feedback 



Fig. 13-4. A typical electronic system. 



The biological response, or the response of a physical instrument used 

 in a biological experiment, must be converted into some kind of electri- 

 cal signal. Any device which does this is called an input transducer. It 

 accepts information and converts it into some usable electrical form. In 

 the simplest system the input transducer feeds directly into an output 

 transducer, such as a meter. More commonly, the electrical signal is 

 amplified. The system then becomes more complex because the amplifier, 

 and perhaps the other components, require a power supply to furnish 

 a number of d-c and a-c voltages. The output of the system may be 

 recorded, or it may be used to control, or both. A complete system is 

 diagrammed in Fig. 13-4. Each major component is considered in more 

 detail in a succeeding section. 



Input transducers 



Input transducers which respond to a variety of signals are available. 

 These are admirably covered in the book by Lion.^ It is a shame that the 

 other components of systems are not yet treated so well. 



Mechanical Transducers: Input transducers in this class respond to 

 movements, changes in pressure, or other mechanical changes. A simple 



^ Kurt S. Lion, Instrumentation in Scientific Research: Electrical Input Trans- 

 ducers (New York: McGraw-Hill Book Company, Inc., 1959). 



