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BELL SYSTFJf TECHNICAL JOURNAL 



The actual cavity is illustrated in Fig. 1. This is somewhat more com- 

 plicated than a half wave line, but the mode of resonance is essentially 

 the same. The variable capacitor utilizes a thin-walled copper cup which is 

 movable vertically. This cup fits rather closely inside, and is coaxial with, 

 a cvlindrical hole in the main bodv of the cavitv. It forms the center 



SPEAKER TYPE 

 MAGNET 



SUPPORT 

 SPRINGS 



PAPER TUBE 



WAVEGUIDE 

 OUTPUT 



ANODE '■ 

 CONNECTION 



I'"ifi. 1. — Construction of the oscillator. 



conductor of a low impedance coaxial line approximately one-fourth 

 wavelength long, so that in this frequency range it is eflfectively short- 

 circuited to the cavity wall. Vertical motion of the cup is therefore roughly 

 equivalent to moving the end wall, thereby changing the capacitance 

 between the wall and the center conductor of the main cavity. The reso- 



