THERMIONIC VACUUM TUBES 



93 



of directive radio in which a metallic mirror was used to reflect the 

 transmitted signals. 



Very different means of producing high frequencies have been used 

 by R. Whiddington, 41 Barkhausen and Kunz, 42 and by Gill and Mor- 

 rell. 43 In some cases they employ tubes having considerable residual 

 gas. The frequencies produced depend on the relative voltages ap- 

 plied to the grid and plate. Probably the best explanation of this 

 phenomenon has been given by Gill and Morrell. Frequencies higher 

 than 3X10" cycles per second have been reported. 



The most accurate way of measuring these high frequencies is by 

 observing the length of standing waves produced on a parallel wire 

 system. The constancy of the vacuum tube generator, compared 

 with spark ocsillators, combined with the fact that the sharpness of 

 resonance in a parallel wire circuit is comparable with that in ordinary 

 radio circuits, makes it especially adaptable to measurement pur- 

 poses. It may be used, for example, to measure small inductances 

 and capacities or to determine the dielectric constant of liquids. 

 Many of the corrections necessary when a damped source is used are 

 eliminated. 



55. The Mechanically Coupled Oscillator. In addition to the types 

 of oscillators described above where the frequency is determined by 



^Eb 



Fig. 68a 



inductance and capacity, we may have oscillators in which the fre- 

 quency is governed by a mechanical system such as a pendulum or a 

 tuning fork. 44 An example is shown in Fig. 68a. The two coils 



41 R. Widdington, Radio Rev., 1, Nov., 1919. 



43 Barkhausen and Kunz, Phys. Zs., Jan, 1, 1920. 



« Gill and Morrell, Phil. Mag., 4-1, July, 1922. 



"See Eckhardt, Karcher and Reiser, /. 0. S. A. & R. S. I., Vol. 6, p. 948, 1922; 

 Eccles& Jordan, Phys. Soc. Proc. 31, Aug., 1919 and Phys. Soc. Proc. 32, Aug., 1920; 

 Abraham & Bloch, ]. d. Physique, Vol. 9, July, 1920. 



