A SELECTIVE HOT-WIRE MICROPHONE. 
425 
aperture of a resonant microphone, and the connection with the trumpet was such 
as to leave unchanged the resonant note of the microphone by modification of its 
orifice. 
The source of sound was the electrically maintained tuning-fork previously referred 
to and its distance from the trumpet was 50 yards. In order to enhance the source 
of sound the prongs of the tuning-fork were caused to set in resonant vibration the 
air in a glass bottle, and the mouth of the bottle was taken as the position of the 
source. 
To get the zero of bearing, cross-threads were fixed to the trumpet mouth so that they 
coincided with two perpendicular diameters of the mouth. A small sighting-hole was 
drilled in a brass plate covering the narrow end of the trumpet. By looking through 
this aperture and rotating the trumpet until the cross-threads appeared in line with the 
source one was able to observe the zero of bearing on the scale. 
The following table gives readings of the vibration galvanometer for various 
orientations of the trumpet:— 
Bearing in 
degrees. 
Deflection in 
divisions. 
Bearing in 
degrees. 
Deflection in 
divisions. 
0 
48 
50 
36 
5 
48 
55 
34 
10 
47-5 
60 
31 
15 
47 
65 
28 
20 
46 
70 
25 
25 
45 
75 
22 
30 
43-5 
80 
18-5 
35 
42 
85 
15 
40 
40 
90 
11 
45 
38 
It is interesting to note that the intensity of sound at the throat of the trumpet 
increases again as the bearing approaches 180 degrees and gives a maximum. Such an 
effect can be easily observed by fitting a stethoscope to the narrow end of the trumpet 
and listening by ear as the trumpet is rotated. 
When the trumpet is removed and the sound recorded by a microphone alone, an 
estimate of magnification is given. It was found that the ratio 
Maximum deflection with the trumpet 
Deflection without the trumpet 
14-5, 
which is a measure of amplitude magnification, since it has been shown in § 7 that 
deflection is proportional to amplitude. 
