ACOUSTIC IMPEDANCE. 



13 



The "damped impedance" of the receiAer is subtracted vectorially 



from the "free impedance," for a mmiber of successi^-e frequencies. 



The differences, or "motional impechmces" of the receiver are plotted 



on a separate diagram. The ordinary motional-impedance diagram 



of a recei\-er, thus tested in the open, is very nearly a circle passing 



through the origin; i. e., the well known motional-impedance circle. 



The motional-impedance circle of the tested ^ecei^-er is given in Figure 6^ 



55°6 

 Its diameter is 158\ 55!t) ohms; so that the angle ^° = ^ = - 27!8. 



Figure 6. Motional Impedance Circle of Telephone Receiver Used in the 

 Tests. 



The d-c. resistance of this receiver at 20°C is 8G.4 ohms; so that the 

 motional impedance per unit of d-c. resistance, Zo/Ri for this instru- 



