2 INSTRUMENTATION IN SCIENTIFIC RESEARCH 



B. Modifiers: converting an electric signal into another modified 

 electric signal (e.g., amplifier, filter) 



C. Output transducers: converting an electric signal into a non- 

 electrical quantity (e.g., meter, strobotron tube) 



This book deals with input transducers; the standard transducer 

 systems, or "basic solutions," are described, as well as a number of 

 variations of each basic theme. 



General Properties of Instrumentation Elements 



All instrumentation elements are characterized by the following 

 basic criteria: 



A. Input Characteristics 



1 . Type of Input. This is the physical agent acting on the element 

 and producing an output signal. In the case of an input transducer 

 the input can be any physical quantity ; in a modifier or output trans- 

 ducer it may be a voltage, a current, an impedance, or a variation or 

 function of time of these quantities (e.g., time derivative, frequency, 

 phase). 



2. Useful Range of the Input Quantity for Which the Element Can 

 Be Used. The lower limit of the useful input range of an element is, 

 in general, imposed by the instrument error or by the unavoidable 

 noise originating in the element. The upper limit of a useful input 

 level is generally reached when excessive distortion of the signal sets 

 in or when the input signal tends to damage the element. 



3. Effect of the Element upon the Preceding Stage or upon the Object 

 under Investigation. The magnitude of this effect can frequently be 

 expressed by the input impedance of the element or by the amount 

 of force, energy, or power required from the stage preceding the 

 element. 



If the physical mechanism involved in the operation of an element 

 is such that the input quantity actuates, or drives, the element (e.g., 

 if the input quantity is an electric signal and the element to which it 

 is applied is a galvanometer) , a maximum of power is transferred 

 from the preceding stage to the element if the input impedance of 

 the element matches the output impedance of the preceding stage. 

 However, if the input quantity only controls the element (e.g., a volt- 

 age source applied to the control grid of a vacuum tube), impedance 

 matching between the output from the source and the input of the 

 element is not required. 



