The Impedance Concept and Its Application to Problems of 

 Reflection, Refraction, Shielding and Power Absorption 



By S. A. SCHELKUNOFF 



This paper calls attention to the practical value of a more ex- 

 tended use of the impedance concept. It brings out a certain 

 underlying unity in what otherwise appear diverse physical phe- 

 nomena. Although an attempt has been made to trace the history 

 of the concept of "impedance" and many interesting early sug- 

 gestions have been found, reference to these lies beyond the scope 

 of this paper. Apparently, Sir Oliver Lodge was the first to use 

 the word "impedance," but the concept has been developed grad- 

 ually as circumstances demanded through the efforts of countless 

 workers. 



The main body of the paper is divided into three parts: Part I, 

 dealing with the exposition of the impedance idea as applied to 

 different types of physical phenomena; Part II, in which the 

 general formulae are deduced for reflection and transmission co- 

 efficients; Part III, presenting some special applications illustrating 

 the practical utility of the foregoing manner of thought. 



THE term "impedance" has had an interesting history, in which 

 one generalization has suggested another with remarkable rapid- 

 ity. Introduced by Oliver Lodge,^ it meant the ratio V/I in the 

 special circuit comprised of a resistance and an inductance, / and V 

 being the amplitudes of an alternating current and the driving force 

 which produced it. This was soon extended to the somewhat more 

 general circuit consisting of a resistance, an inductance coil and a 

 condenser.- The usage did not develop much further until the use of 



1 Dr. Oliver Lodge, F.R.S., "On Lightning, Lightning Conductors, and Lightning 

 Protectors," Electrical Review, May 3, 1889, p. 518 



2 It is interesting to note that the first impulse was to introduce a new word 

 rather than to extend the meaning of the old term. Thus in 1892, F. Bedell and 

 A. Crehore write as follows: "From the analogy of this equation to Ohm's law, we 



see that the expression -» /i?2 _|_ (_ j^^ j is of the nature of a resistance, and 



is the apparent resistance of a circuit containing resistance, self-inductance and 

 capacity. This expression would quite properly be called 'impedance' b ut the term 

 impedance has for several years been used as a name for the expression Vi?^ + XW, 

 which is the apparent resistance of a circuit containing resistance and self-inductance 

 only. We would suggest, therefore, that the word 'impediment' be adopted as a 



name for the expression ^/i?2 -\- (— Leo] which is the apparent resistance of a 



circuit containing resistance, self-induction and capacity, and the ter m impedanc e be 

 retained in the more limited meaning it has come to have, that is Vi?^ + LW, the 



17 



