vi PREFACE 



the result of expertness in mathematics only, nor of acrobatics in 

 experimental techniques, but are often the consequence of a clear 

 understanding of the physical principles or mechanisms underlying 

 a process. The research worker or engineer frequently scans the 

 field for such physical principles and creates a new solution by some 

 kind of analogy. By furnishing the reader with a simple description 

 of the physical mechanisms underlying each method, I hope to 

 render him a service which will facilitate the finding of new and 

 better methods and instruments. 



The question has been raised whether a book of such general 

 scope is justifiable or whether it would not be more reasonable to 

 provide each science, such as the physical, the biological, or the 

 engineering sciences, with its own book containing only those 

 methods and systems that are of primary interest in each science. 

 Many years of teaching research methods and instrumentation to 

 students and research workers in different fields have convinced me 

 that a specialized approach is of limited value, that the restriction 

 to research methods in any one field leads to unnecessary duplica- 

 tion of work (the existing literature furnishes ample proof of this), 

 and that a broad knowledge of the entire field of methodology is 

 demanded, in particular by the experienced research worker. 



Of course, it would be impossible (and useless) to describe the 

 innumerable methods and instruments that are being used, even if 

 one limits oneself to the electrical systems only. However, all 

 electrical instruments are composed from elements, such as input 

 transducers, amplifiers, filters, integrators, output transducers, etc. 

 These elements recur, in one form or another, in many applications ; 

 since their number is limited, they can be adequately discussed 

 without the danger of superficiality. They may be synthesized into 

 complete systems in accordance with the well-known principles and 

 laws that govern the behavior of such physical systems. I have 

 tried to furnish enough data for the reader to make an intelligent 

 choice in the selection and combination of the elements. The final 

 synthesis, however, is frequently a matter of engineering art, or the 

 art of the research worker, and cannot be taught. 



The level of presentation is chosen for those who speak the 

 language and know the concepts of elementary physics and have at 

 least a moderate mathematical knowledge. The physics of each 

 method has been given preference over the technical means. Mathe- 

 matical derivations are omitted; where the derivation is not obvious 

 it may be found in the references. 



In the selection of the material, I have limited myself to those 



