94 



BELL SYSTEM TECHNICAL JOURNAL 



LpLg are inserted in the plate and grid circuits of the tube. Variations 

 in the plate current through the coil L P impress forces on the tuning 

 fork which result in its motion. This motion of the fork causes 

 variations in the magnetic field through L g and induces a varying 

 voltage on the grid. With the proper coupling, sustained oscillations 

 result having a period very nearly that of the tuning fork. 



The electrically driven tuning fork described above constitutes a 

 very satisfactory source of either sound or electromotive force. Hor- 

 ton, Ricker and Morrison 45 have made improvements which make it 



Fig. 68b 



constant in amplitude and frequency to six parts in a million over very 

 long periods of time. 



An entirely different form of mechanical coupling has been used by 

 Cady. 4s The circuit is shown in Fig. 68b. It makes use of the 

 piezo-electric effect and mechanical vibrations of a crystal. Varia- 

 tions in the plate current in the tube 9 cause a voltage change across the 

 resistance 5. This is communicated to a crystal AB such as quartz by 

 means of the plates 2 and 4. A transverse electric field applied to 

 such a crystal causes a change in its length. If this electric field be 

 periodic, compression waves will travel along the crystal with a 

 velocity depending on its density and elastic properties. These 

 waves will, in turn, cause a varying electric field between plates 1 

 and 3 which may be communicated to the grid of the tube 7, amplified 

 by 8, and finally transmitted to tube 9. This provides conditions 

 for sustained oscillations having a frequency which is roughly in- 

 versely proportional to the length of the crystal . 



Cady describes oscillators ranging in frequency from 3 X 10 4 to 10 6 , 

 and states that the frequency is constant to about one part in 10,000. 

 The effect of temperature change is not great. 



45 Journal of A. I. E. E., 1923. 



46 Cady, Proceedings of I. R. E., Vol. 10, No. 2, April, 1922. 



