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DR. TUCKER AND MR. PARIS: A SELECTIVE HOT-WIRE MICROPHONE. 
in resistance due to an average cooling of the grid, and a periodic change due to the 
to-and-fro motion of the air. Two methods of using the microphone are described :— 
(i) A Bridge method, depending on the steady drop in resistance ; and 
(ii) An Amplifier method which makes use of the periodic resistance changes. 
Curves are given showing the sharpness of resonance as measured by the Bridge 
method. 
The various factors affecting the sensitivity of the microphone are discussed. The 
most important, from a practical point of view, is the variation of the sensitivity 'with 
the heating current of the grid. It is found by experiment that the sensitivity always 
increases as the heating current is increased. In the case of the Bridge method, it is 
found that the steady resistance change produced by a sound of given intensity is a 
linear function of the temperature of the grid above its surroundings measured on the 
platinum scale. 
The results of experiments on the cooling of the grid by low velocity air-currents are 
described. From these results it is deduced that the principal resistance changes to 
be expected when the grid is cooled by an oscillatory air-currents are :— 
(1) A steady drop due to an average cooling ; 
(2) A periodic resistance change at the same frequency as that of the sound ; and 
(3) A periodic resistance change of frequency twice that of the sound. 
All these effects are found in practice. 
Further deductions are that the steady change of resistance is proportional to the 
intensity of the sound, while the periodic resistance change in (2) is proportional to the 
amplitude. These conclusions are confirmed by experiment. 
A description is given of the effect to be observed when the microphone is tilted at 
various angles, and the observed facts are compared with what would be expected from 
the results of experiments with steady air-currents. 
Finally, an account is given of some experiments which exemplify the use of the 
Hot-Wire Microphone for observing the intensity and distribution of sound. 
The work described in this paper forms part of an investigation commenced in the 
Munitions Inventions Department, and continued later at the Signals Experimental 
Establishment, Woolwich. 
In conclusion, the authors wish to express their indebtedness to the Chief Experimental 
Officer of this Establishment for the interest which he has taken in the progress of the 
work, and for the facilities which they have received for carrying out experiments. 
PRESENTED 
r- A n IQ 0 \ 
