CONDENSER DRIVING SYSTEM 107 



Combining equations 6.23 and 6.28 



6.29 



M 



_ 4A^2^iM 

 "^EM — ~7^ 6.30 



where %em = motional impedance, in abohms, and 



%MT = total mechanical impedance including the armature with 

 reference to a point on the armature directly over one of 

 the pole pieces. 

 The entire reluctance is assumed to reside in the air gap and equation 

 6.30 may be written, 



4A^U,2 



6.31 



a^ZMT 



Equation 6.31 is essentially the same as equation 6.19 for the reed arma- 

 ture type and is similar to equation 6.6 for the electrodynamic system. 



When the armature is displaced by the current, means must be provided 

 for returning the armature to the equilibrium position. Due to the large 

 magnetic forces, the stiffness of the centering system must be relatively 

 large. 



This driving system is used for loud speakers, galvanometers, for motion 

 picture film recording galvanometers and for facsimile printers. 



In actual practice it appears very difficult to reduce the stiffness suf- 

 ficiently so that the resonance of the system will occur below 100 cycles. 

 Therefore, when this driving system is used for a loud speaker the response 

 will fall off quite rapidly below the resonance frequency. 



6.4. Condenser Driving System. — A condenser driving system is a 

 driving system in which the mechanical forces result from electrostatic 

 reactions. Consider the system of Fig. 6. ID, consisting of a vibrating 

 surface moving normal to its plane and separated from a fixed conductor. 

 The force, in dynes, between the plated per unit area is 



where e = voltage between plates, in statvolts, and 



X = distance between the plates, in centimeters. 

 Assume that the polarizing voltage is e^ and that the alternating voltage 



