134 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



changed the stationary sound system when he walked through the 

 room. Professor Sabine, in some experiments not yet pubhshed, had 

 previously noticed the effect of a shift in the stationary wave system 

 in modifying the draft of power by an electrically-driven tuning fork 

 kept vibrating at constant amplitude. In accordance wuth the pres- 

 ent experiments and as Professor Sabine previously suggested in 



Figure 7. Circular graph for bipolar Bell receiver /?t with 0.3 volt at 

 terminals. Diameter 103 ohms. Depression angle (2/3) 70.5°. wq = 4885 

 radians per sec. a = 200. Small circles observed. Internal ring numbers 

 computed. 



conversation with one of the writers, the phenomenon is seen to have 

 its explanation in the change of resistance and reactance of the coil 

 of the fork due to the variously affected motion of the fork. 



Circular Graphs of Motional Impedance. — ^ A very interesting 

 result is obtained by plotting the motional reactance of a telephone as 

 ordinate against the motional resistance as abscissa. The result is a 

 point on the R X plane, and the point is different for different values 

 of the angular velocity used in the measurement. The locus of this 

 point, as the angular velocity is varied, is a circle passing through the 

 origin ; that is, through the point of zero motional resistance and zero 

 motional reactance. Stated otherwise, if the motional impedance, 

 (R' — i?) + j{X' — A'), is plotted vectorially from a point as origin, 



