460 KENNELLY AND AFFEL. 



The effect of the air chamber, as compared with an open diaphragm, 

 was to lower the resonant frequency sHghtly, and to reduce the circle 

 diameter. 



In order to ascertain the influence of the ordinary atmospheric 

 pressure upon the characteristics of the diaphragm, a bipolar receiver 

 was suspended in the glass bell-jar of an air-pump, the two wires to the 

 receiver being carried through a seal at the top of the jar. One mo- 

 tional-impedance circle was obtained with full atmospheric pressure, 

 and another after the pressure had been reduced to about 1 cm. of 

 mercury. The two circles are shown in Figure 22. It will be seen 

 that the removal of atmospheric pressure decreased the resonant fre- 

 quency and enlarged the circle diameter. The results are given in the 

 following Table: 



TABLE VIII. 



The Effect of the Atmosphere on Receiver Characteristics. 

 Receiver C. 



Assuming that the equivalent mass m remained unchanged, the 

 equivalent resistance r diminished in vacuo by about 30 per cent. 

 The power of the receiver as a sound-producing device is theoretically 

 limited to (x)V or 101 (.r)^ abwatts. Owing to the fact that the 

 underside of the diaphragm is cut off from free access to the air, only 

 part of this power can be actually utilised for sound production in air. 



At certain frequencies near resonance, the Rayleigh bridge balance 

 becomes very sensitive to variations in atmospheric pressure. From 

 an examination of the balance at these frequencies, it was easy to 

 ascertain whether air was leaking into the bell jar. It seems likely, 



