ACOUSTIC IMPEDANCE. 



11 



Vreeland oscillator in some of the tests, and a triode vacuum-tube 

 oscillator in others. The resistance R was adjusted to give an alter- 

 nating current of 3 rms. milliamperes to the bridge. In some tests, 

 this current was raised to 4 milliamperes. Half this current passed 

 through the receiver Z, and the other half through the adjustable 

 anti-inductive resistance ;•, and variometer /. The two arms a and b 

 of the bridge are of equal anti-inductive resistances. The current- 

 detector, or galvanometer, was a pair of medium-resistance head 

 telephones. 



Figure 4. Electrir- Connections of Raylfigh Bridge for measuring the 

 Motional Impedance of the Telephone Receiver Z. 



The'measurements were all made with approximately constant a-c. 

 strength through the measured receiver Z. The measured iniped- 

 ance of the receiver did not, however, vary materially over a small 

 range in current strength; so that it was not important to keep 

 the current strength ^'e^y nearly constant. The current was ascer- 

 tained and adjusted by means of a static voltmeter V, bridged across 

 an anti-inductive resistance s of 3000 ohms. In some of the tests, a 

 thermojunction and d-c. gah'anometer were substituted for s and V. 



