The Impedance Concept and its Application to Problems of Reflection, Refraction, Shielding and Power Absorption

HE term "impedance" has had an interesting history, in which one generalization has suggested another with remarkable rapidity. Introduced by Oliver Lodge,1 it meant the ratio V/I in the special circuit comprised of a resistance a n d an i n d u c t a n c e , I a n d 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.2 The usage did not develop much further until the use of 1 Dr. Oliver Lodge, F.R.S., " O n Lightning, Lightning Conductors, and Lightning Protectors," Electrical Review, M a y 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: " F r o m the analogy of this equation to Ohm's law, we see that the expression -^g* -)- ^ 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' but the term impedance has for several years been used as a name for the expression Vi?2 + L 2u 2, 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 ^J: which is the apparent resistance of a circuit containing resistance, self-induction and capacity, and the term impedance be retained in the more limited meaning it has come to have, that is V-R2 + LW, the 17 18 BELL SYSTEM TECHNICAL JOURNAL complex quantities, which had begun early in the nineteenth century among mathematicians, was popularized among engineers by Kennelly and Steinmetz.