A SELECTIVE HOT-WIRE MICROPHONE. 
407 
and the velocity of the fall (or rise) of the surface of the water in D. The latter was 
found by timing the movement of the level of the water in the gauge (G). The maximum 
velocity of the fall or rise in D required in the experiments was 0 -09 cm. per second, 
corresponding to a velocity of 6 cms. per second for the air passing the grid. 
The microphone formed one arm of the Wheatstone’s Bridge shown in fig. 4. The 
method of taking an observation was as follows. The balancing resistance R was given 
some prearranged value about equal to the sum of the resistance of the grid (carrying 
its normal current), the milliammeter and the variable resistance p set at 10 ohms. 
The resistance p could be varied by steps of 0 T ohm from 0 to 20 ohms. With the tap (T) 
closed, the bridge was then balanced by adjusting the rheostat (Rh), that is, by altering 
the heating current until the total resistance of the microphone, milliammeter and p 
was equal to R. The tap (T) was then opened and adjusted to give an air-current past 
the grid of approximately the required velocity. The exact velocity of the air-current 
was determined by timing with a stop-watch the fall or rise of the level of the water in 
G for 1, 2 or 3 cm., according to the magnitude of the velocity employed. The change 
in the resistance of the grid was determined at the same time by increasing or decreasing 
the resistance p so as to restore the balance. The resistance-change was determined 
to 0 -01 ohm by noting the deflection which remained after the bridge had been balanced 
as nearly as possible by altering p . The effect of a forced air-current on the resistance 
of the grid was thus determined under the condition that the electric current carried by the 
grid remains constant. 
The results of one experiment are shown in the following table, which gives U, the 
impressed air-current, in centimetres per second, and oR the change in the resistance of 
the grid. The air-current U is taken as positive when it flows into the container—in 
this case vertically downwards. 
Microphone Grid A1079. Heating current 28 -5 milliamperes. Resistance of grid 
when impressed air-current is zero = 270-8 ohms. 
u. 
cm. per sec. 
SR. 
ohms. 
U. 
cm. per sec. 
SR. 
ohms. 
-4-12 
-9-73 
+0-27 
+0-46 
—3-54 
-7-76 
+0-53 
+0-50 
-3-13 
-7-46 
+0-55 
+0-56 
-3-04 
-6-83 
+0-90 
+0-73 
-2-20 
-4-34 
+1-33 
+0-79 
-2-07 
-4-18 
+1-40 
• +0-74 
-1-65 
-2-91 
+1-97 
+0-47 
-1-33 
—2 • 15 
+1-99 
+0-43 
-0-74 
— Ill 
+2-40 
+0-10 
-0-48 
-0-76 
+2-67 
-0-37 
-0-45 
-0-61 
+3-61 
-1-92 
0 
0 
+4-30 
-3-36 
3 K 2 
