A SELECTIVE HOT-WIRE MICROPHONE. 
415 
The general procedure followed in the experiment was similar to that already 
described. The Wheatstone’s Bridge was replaced by an Amplifier of the Army pattern 
known as “ C Mark II.,” the output terminals being connected to a Campbell vibration- 
galvanometer tuned to respond to 250 vibrations per second. The source of sound was 
the same as before. It was not necessary in this case to take a zero reading for each 
position of the microphone. 
The results of two experiments, carried out on different occasions and with different 
strengths of the source, are given in the following table:— 
Experiment A. 
Experiment B. 
Distance in 
Deflection in 
Distance in 
Deflection in 
feet. 
centimetres. 
feet. 
centimetres. 
8 
11-4 
20 
10-1 
12 
9-0 
24 
7-3 
16 
7-2 
28 
6-1 
20 
6-0 
32 
5-6 
24 
5-4 
36 
4-8 
28 
5-1 
40 
4-4 
32 
4-6 
44 
4-2 
36 
4-2 
48 .. 
4-1 
40 
3-8 
52 
3-9 
44 
3-2 
56 
3-7 
48 
3-0 
60 
3-0 
• 
64 
2-7 
The curves A and B (fig. 12) show the results of plotting the observations in 
Fig. 12. 
3 L 2 
