ACOUSTIC IMPEDANCE. 



13 



The "damped impedance" of the receiver is sul)tracted vectorially 

 from the "free impedance," for a number of successive frequencies. 

 The differences, or "motional impedances" 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 receiver is given in Figure 6. 



55°6 

 Its diameter is 15S\ 55?6 ohms; so that the angle j3° = ^^ = — 27?8. 



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



Tests. 



The d-c. resistance of this recei^-er at 20°C is 80.4 ohms; so that the 

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



