300 BELL SYSTEM TECHNICAL JOURNAL 



with an electromotive force. Under other conditions, it may behave 

 as a circuit made up of inductance and capacity in parallel with an 

 electromotive force.^ At still other frequencies it may present to a 

 source a positive (inductive) reactance or a nej^ative (capacitive) 

 reactance. This makes possible circuit elements which may be com- 

 bined to form various filter or network equivalents. We may have, 

 therefore, from wave guides frequency selection by either or both of 

 two fundamentally different properties. 



Radiation 



Discontinuities in wave guides, particularly those in which no shield 

 is present, tend toward losses by radiation. In the case of a hollow 

 conducting pipe radiation issues from the open end much the same as 

 sound waves from a hollow^ tube. It has been possible to expand the 

 ends of these pipes into horns, thereby obtaining effects very similar 

 to those common in acoustics. Such an electrical horn not only pos- 

 sesses considerable directivity but it may also provide a moderately 

 good termination for the pipe to which it is connected. In so doing its 

 function is probably quite analogous to that of a true acoustic horn 

 which provides an efficient radiating load for its sound motor. 



Some Apparatus and Methods Used in Wave 

 Guide Studies 



It is obvious, of course, from the very nature of guided waves that 

 the apparatus and methods must be rather different from the more 

 common electrical methods. This difiference is such that an adequate 

 description would require more space than is here available. How- 

 ever, for purposes of completeness a few of the more interesting and 

 fundamental aspects of the experimental side are included below. 

 For the most part this description will center around the Ih t\^pe of 

 wave. (See Fig. 1 above.) 



The Simple Resonant CJiamber 



In much the same way that the simple tuned circuit containing 

 localized inductance and capacity is fundamental to the radio art so 

 also is the simple resonant cavity fundamental to wave guide work. 

 Although it may assume a variety of forms, one of the more obvious 

 is a short piece of cylindrical wave guide, preferably of hollow metal 

 pipe bounded by a piston and an iris diaphragm as shown in Fig. 11. 



2 In pursuint; ihis work it has been convenient at times to refer not onh- to circuit 

 analo^'ues but also to optical anrl also acoustical analogues. This has been due in part 

 to the lack as \ct of an adequate vocabulary and in part to the h\brid nature of (he 

 subject at hand. 



