A SELECTIVE HOT-WIRE MICROPHONE. 
401 
Since f = —, F being the variable part of the pressure at the mouth of the 
P 
resonator, the actual “ magnification ” of the displacements and velocities obtained by 
using the resonator is seen to be 
y c 
2K 
and is determined by the ratio of the conductivity of the neck to the damping factor K. 
Since, when c is constant, K is found to be larger the higher the pitch of the resonator 
employed, the efficiency of the microphone for the higher notes is correspondingly 
diminished. 
As a numerical example, we may quote the case of the resonator used to obtain the 
curve in fig. 5, for which N = 240 and Iv = 38 -5, so that, putting V = 33760 cm./sec. 
and c = 0 -13 cm., the “ magnification ” is about 57; while for a resonator of pitch 112, 
and K = 22*2, the “ magnification ” is about 100. It may be noted here for future 
reference that if Iv = 38 -5, and the amplitude of the sound outside the resonator is 
1 -27 x 10 " cms., i.e., Rayleigh’s value for the minimum amplitude audible when 
N = 256, then the maximum velocity in the neck of the resonator will be about 
0-0116 cm. per sec., and that, even if the amplitude is two hundred times the above value, 
the maximum velocity in the neck will still be less than 2 -5 cms. per sec. 
One of the most important factors in determining the efficiency of a resonator is the 
rigidity of the walls of the container. This was well shown by the following 
experiment. 
A cylindrical resonator of rolled veneer was tested and found to respond to the same 
frequency as a brass resonator of the same volume with the same orifice. The resonant 
note was 79 vibrations per second. Experiment showed that its degree of response 
(measured with a Wheatstone’s Bridge) was only one-third of that of the brass one, the 
conditions being as nearly as possible the same in both cases. The resonance curve 
for the veneer resonator showed that the appropriate value of Iv was about 35. 
We have next to consider some points in connection with the sensitivity of the grid. 
Almost the first problem that arises in constructing a microphone of this pattern is 
the choice of a suitable diameter for the wire. In the first experiments that were made 
with microphones of this type the diameter of the wire used was 0 -0015 cm. It was 
found, however, that better results were obtained with finer wire, and from time to time 
experiments have been carried out with wire of various diameters down to 0 -0002 cm. 
These experiments showed that the finer the wire the greater was the sensitivity (more 
especially for high-pitched notes), but that the increased sensitivity obtained with very 
fine wires was very often counter-balanced by their extreme fragility, which rendered 
them unsuitable for anything but very special purposes. Finally, a wire of diameter 
about 0 -0006 cm. has been adopted as being sufficiently sensitive, and at the same time 
not too fragile to prevent its being employed in ordinary out-of-door experiments. 
The sensitivity is most easily controlled by altering the heating current. No matter 
