signal is measured by a vacuum tube voltmeter. The cavity 

 must now be tuned to resonance manually. However, it is 

 possible to "probe" the cavity with a small captured bubble 

 and ascertain when the desired mode is obtained, by deter- 

 mining the position of the nodal lines with the bubble. This 

 procedure would almost invariably stop the sing-around 

 circuit in figure 4A from, oscillating. The frequency can 

 be changed in small increments by the cycles increment 

 dial on the generator and the Q factor can be evaluated in 

 this fashion, when the losses are large. 



Mi r- 



1 II 



MAIN DIAL 

 INCREMENT DIAL 



//////////////////////////////////// 



Figure 4. Diagrams of measuring systems. A, preliminary 

 system; B, system employed in final tests. 



19 



