Oscillations in an Electromechanical System 



By L. W. HUSSEY and L. R. WRATHALL 



Experimental results are given on an oscillating electromechanical system 

 in which, under a single frequency impressed electromotive force, mechanical 

 vibrations are sustained at a freciuency near the resonant frequency of the 

 mechanical system and electrical oscillations at the difference between the 

 frequency of the mechanical vibration and that of the impressed force. 



The system is the one studied analytically by R. V. L. Hartley in an 

 accompanying paper. Its performance conforms to the principal operating 

 features predicted in his analysis. 



IN AN accompanying paper ^ an analytic investigation is made of a 

 system involving a non-linearity in the coupling between an elec- 

 trical and a mechanical system. The electro-mechanical system under 

 discussion is., in its simplest form, a condenser, with one plate sharply 

 resonant mechanically, a generator, and an impedance, all connected 

 in series. If the charge on the condenser is q, there will be a force on 

 the mechanical system proportional to (f. While the mechanical 

 system and the electrical system involved are individually linear, there 

 is a non-linearity in this electrostatic coupling, and hence the pos- 

 sibility exists of mechanical and electrical vibrations at other fre- 

 quencies than the impressed frequency. On this basis the possibility 

 of the generation of a mechanical vibration, not at a harmonic of the 

 impressed electromotive force, and electrical currents at the difference 

 between the frequency of the mechanical vibration and that of the 

 impressed electromotive force was predicted by the analysis. 



That the phenomenon discussed can occur was first verified by Mr. 

 Eugene Peterson. A condenser microphone was given a mechanical 

 resonance at 600 cycles per second, by cementing a small metal ball to 

 the center of the diaphragm. An alternating electromotive force at 

 2200 cycles per second was impressed and the system given a series 

 resonance at the difference frequency, 1600 cycles per second, by 

 means of an inductance. When the impressed voltage was increased 

 beyond a critical value mechanical vibrations suddenly built up and 

 current of the difference frequency, larger in amplitude than the 

 current of the impressed frequency, appeared in the electrical system. 

 The same result was obtained using a prong of a tuning fork as the 

 vibrating plate. 



1 "Oscillations in Systems with Non-Linear Reactance" by R. \'. L. Hartley, in 

 this issue of the Bell Sys, Tech. Jour. 



441 



