96 BELL SYSTEM TECHNICAL JOURNAL 



the smaller we make the inductance in the tuned circuit, and the larger 

 we make the capacity, the frequency being kept constant, the greater 

 will be the range of frequencies over which the condition, required by 

 (8), that the series reactance of the tuned circuit be zero, is satisfied to 

 an approximation sufficiently good for practical purposes, and hence 

 the less critical will be the adjustment of the stabilizing impedance. 



For the data in Figs. 20 and 21a reversed feed-back type of oscilla- 

 tor was employed, having an elementary circuit similar to that of Fig. 

 12. A grid leak was placed in parallel with the stabilizing capacity, 

 but care was taken to see that the resistance of the leak was always so 

 high that its value did not affect the frequency. This was done by 

 using a variable resistance and increasing its value until the frequency 

 no longer changed. A large grid leak has always been found advan- 

 tageous in securing constancy of frequency, but the size of the leak 

 reaches a practical limitation determined by the time constant which 

 produces the familiar "blocking." 



The plate battery potential was fed through a choke in series with 

 the plate inductance coil, the combination of choke and B battery 

 being thoroughly by-passed with a large condenser. 



For the purpose of checking the size of the stabilizing capacity to 

 find its agreement with theory, an indirect method was used. The 

 theory requires that 



Lr( 1 



as shown in Fig. 12. The operating frequency was one megacycle, 

 which made a direct measurement of the coupling coefficient, k, some- 

 what awkward, so that a method based on the "unity coupling" con- 

 cept was employed. Thus from (26) 



k'^co-LiLi — (jiL\ I oiL^ 7T ) (37) 



may also be used for determining G- The primary, Li, of the coil was 

 connected through an impedance to a source of e.m.f. of one mega- 

 cycle, and a vacuum tube voltmeter was placed across it. The con- 

 denser, d, was placed directly in parallel with the secondary, L^. 

 With this arrangement the impedance looking into the primary is 



Z = iCioLii -f- 



i ( C0L2 7^ 



C0C4 



unless points very near the resonance point are considered. This last 



