A SELECTIVE HOT-WIRE MICROPHONE. 
419 
against 0, and demonstrates clearly that the amplitude of the fundamental vibration 
passes through a minimum when 0 is about 125°. 
An application of the experimental method of investigation described in § 6 leads 
to some interesting results. For example, an experiment was performed to determine 
the relation between the resistance R of the grid and the impressed velocity U of the 
air-current when 0 = - . The results are given in the following table. The value of 
2 
0 indicates only the tilt of the axis of the container and neck, and does not necessarily 
mean that the plane containing the loops is exactly vertical. 
Microphone Grid A1079. Heating current 28 -5 milliamperes. Resistance of grid 
when impressed air-current is zero = 266 -85 ohms. 
u. 
era. per sec. 
SR. 
ohms. 
U. 
cm. per sec. 
SR, 
ohms. 
-5-54 
-9-25 
+ 1-04 
+0-002 
-4-22 
6-21 
+1-11 
—0-02 
-4-15 
-5-64 
+] -58 
-0-31 
-2-98 
-2-92 
+2-15 
-0-84 
-2-32 
-1-78 
+3-48 
-2-99 
-1-36 
-0-93 
+4-49 
-5-46 
-1-17 
-0-70 
+5-11 
-6-51 
-0-64 
-0-34 
These observations are shown graphically in fig. 16. 
