427 
A SELECTIVE HOT-WIRE MICROPHONE. 
rises a deflection is produced in the neighbourhood of resonance. The diagram 
(fig. 20) shows a relation between the pitch of the siren note and the response of the 
trumpet. The amount of resonance is expressed in terms of galvanometer deflection as 
shown by the following table:— 
Frequency 
(vibrations per sec.). 
Deflection 
(divisions). 
Frequency 
(vibrations per sec.). 
Deflection 
(divisions). 
80 
3 
200 
30 
100 
5-5 
220 
49 
120 
7-5 
240 
32 
140 
10 
260 
17 
160 
13 
280 
16 
180 
20 
300 
24 
The table shows an upward curve at the highest frequency, thus indicating approach 
to the next overtone. The maximum at 220 indicates the fundamental resonance 
note. 
By means of the microphone, therefore, the whole properties of a trumpet as a 
receiver of sound can be investigated, both as regards directive action and 
resonance. 
Since by the principle of reversibility we may employ the trumpet as a transmitter 
for any given note, we may also derive its properties as a distributor and magnifier 
of any sound produced at the narrow end. 
This has an obvious application to gramophone trumpets, in which the diaphragm 
acts as the source of sound. 
