302 BELL SYSTEM TECHNICAL JOURNAL 



mounted in a shielded cartridge is coupled to the chamber by a probe 

 wire perhaps a quarter inch long extending through a hole in the wall. 

 Fig. 11^ shows one form of resonant chamber complete with detector. 

 The piston position is read off on a scale and vernier (Fig. 115). 

 Successive positions at which resonance is noted give data for deter- 

 mining velocity of propagation. Chambers of this kind having various 

 diameters were used to verify the velocity ratios shown earlier in this 

 paper. Electrical connection between piston and walls may be had 

 by numerous phosphor bronze fingers, or perhaps by ball bearings 

 distributed in a race around the periphery. Good contact is not 

 always essential. In fact, fair work may sometimes be done with a 

 loosely fitting piston or even an insulated piston. 



Resonant chambers may be activated merely by placing them within 

 a foot or two of a source of waves such as a Barkhausen oscillator. 

 Their dimensions must, of course, conform to the wave-length re- 

 quirements as outlined above. Standard 5-inch OD brass pipe having 

 one-sixteenth inch wall has been found satisfactory for the frequency 

 range from 1500 mc. (X = 20 cm.) to 2000 mc. (X = 15 cm.). Any 

 convenient length around 2 feet is appropriate for the variable type 

 of chamber. 



Generators 



One arrangement for generating the Hi type of wave consists of 

 connecting the primary source of waves between diametrically opposite 

 points on the inside of a hollow cylindrical conductor as shown by 

 Fig. 12A. This primary source may consist of a positive grid (Bark- 

 hausen) tube or a magnetron.^ Both have been used successfully to 

 give frequencies up to about 3330 mc. (X = 9 cm.). 



A typical arrangement of such an oscillator is shown in Fig. 12B. 

 The terminals of the spiral grid of the Barkhausen tube are connected 

 to diametrically opposite points through a suitable by-pass condenser. 

 The filament and plate leads enter along a plane perpendicular to that 

 of the grid. Since the grid leads correspond to lines of electric force 

 in the generated wave, the diametral plane perpendicular thereto 

 corresponds to an equipotential. By locating the plate and filament 

 leads in such an equipotential, their presence will not materially 

 afifect the normal field prevailing in the chamber. In the design 

 shown the filament connectors constitute the outside plates of a three- 

 plate by-pass condenser. The third or central plate is a rigid member 

 grounded on the main guide. It connects to the plate of the Bark- 

 hausen tube. Connections to the exterior are had through five 



'"Vacuum Tubes as HiKh-Frequencv Oscillators," M. J. Kellv and A. L. 

 Samuel, B.S.T.J., \ol. 14, p. 97, January 1935. 



