296 BELL SYSTEM TECHNICAL JOURNAL 



that they will be used even more extensively as their possibilities become 

 better appreciated. 



Interest in waveguides has been greatly enhanced by the fact that they 

 brought with them a series of extremely interesting methods of measure- 

 ment, comparable both in accuracy and scope, with similar measurements 

 previously made only at the lower frequencies. This extension of the range 

 over which electrical measurements may be made has contributed also to 

 neighboring ftelds of research. One early application led to the discovery of 

 centimeter waves in the sun's spectrum. Another led to important new infor- 

 mation about the earth's atmosphere. Still another contributed to the study 

 of absorption bands in gases, particularly bands in the millimeter region. 

 Also of great importance was its contribution to our knowledge of the prop- 

 erties of materials for it led at a fairly early date to measurements at higher 

 frequencies than heretofore of the primary constants, permeability, dielectric 

 constant and conductivity — all for a wide array of substances ranging from 

 the best insulators to the best conductors and including many of the so- 

 called semi-conductors. It is because this new art has already attained con- 

 siderable stature and is already showing promise as an educational medium 

 that this book has been prepared. 



CHAPTER I 

 INTRODUCTION 



1.5 Early History of Waveguides 



That it might be possible to transmit electromagnetic waves through 

 hollow metal pipes must have occurred to physicists almost as soon as the 

 nature of electromagnetic waves became fully appreciated. That this might 

 actually be accomplished in practice was probably in considerable doubt, 

 for certain conclusions of the mathematical theory of electricity seemed to 

 indicate that it would not be possible to support inside a hollow conductor 

 the lines of electric force of which waves were assumed to consist. Evidence 

 of this doubt appears in Vol. I (p. 399) of Heaviside's "Electromagnetic 

 Theory" (1893) where, in discussing the case of the coaxial conductor, the 

 statement is made that "it does not seem possible to do without the inner 

 conductor, for when it is taken away we have nothing left on which tubes 

 of displacement can terminate internally, and along which they can run." 



Perhaps the first analysis suggesting the possibility of waves in hollow 

 pipes aj)peared in 1893 in the book "Recent Researches in Electricity and 

 Magnetism" by J. J. Thomson. This book, which was written as a sequel 

 to Maxwell's "Treatise on Electricity and Magnetism," examined mathe- 

 matically the hypothetical question of what might result if an electric charge 



