VIBRATING TELEPHONE DIAPHRAGMS. 



439 



Torsion-Pendulum Model for Illustrating Motional 

 Velocity Phenomena. 



A psychological obstacle to the use of the motional-velocity 

 circle conceptions, in their abstract quality, and remoteness from 

 concrete apprehension. Thus, in the case of the telephone receiver, 

 its motional-velocity circle is obtained through the medium of the 

 motional-impedance circle, as determined from electrical measure- 



Amplitude Graph K, Velocity Circle K, 



.2 A ■€ .i 1.0 rad, .2 .■* e e jo rad/sec 



I I I I I I I I I 1 1 1 



W<\ 



TL 



Ul 



TL 



=bK, 



Fig. i8. Multiple Coupled Torsion Pendulum Model. 



ments. It is therefore a great advantage to be able to construct a 

 motional-velocity circle from direct observations on a simple dynam- 

 ical model. 



A useful and very simple dynamical model for illustrating the 

 conditions of diaphragm vibration and the resulting motional-im- 

 pedance circle has been designed and constructed as indicated in 

 Fig, i8. It consists of a hollow brass cylinder K^ which, in the 

 particular model used, has a mass of 1,200 gm., a radius of gyration 

 of 2.48 cm., and a moment of inertia of 7,470 gm.-cm.^. This is 

 suspended by a brass wire of diameter 0.76 mm., of a length ad- 



