A SELECTIVE HOT-WIRE MICROPHONE. 
397 
A typical example of a curve obtained in this way is shown in fig. 5, the natural 
pitch of the microphone being 240 vibrations per second. 
Fig. 5. 
In order to obtain reliable resonance curves it was found necessary to make observations 
out of doors. When the experiments were performed indoors the results were in nearly 
all cases vitiated by the setting up of stationary waves in the room containing the 
apparatus. 
Experiments are described in a later paragraph (§ 7) which show that—within limits— 
the change of resistance of the grid is proportional to the square of the amplitude (and 
therefore to the energy) of the vibration in the neck of the resonator, when the pitch 
of the stimulating sound remains constant. The influence of a change in the pitch of 
the sound upon the resistance change of the grid (apart from its effect on the response of 
the resonator) has not yet been investigated. In dealing with the resonance curves, 
where in any particular experiment we are concerned only with a comparatively narrow 
range of frequencies, we shall regard the deflection of the galvanometer as being 
proportional to the vibrational energy in the neck. Precautions were of course taken 
to ensure that the deflections were proportional to the changes in resistance suffered 
by the grid. 
As stated above, the source of sound used in these experiments was a modified form 
of Seebeck’s siren. It consisted of a heavy circular brass plate pierced with a ring of 
3 i 
VOL. CCXXI. 
A. 
