412 
I)R. W. S. TUCKER AND MR. E. T. PARIS ON 
(i) That the steady resistance change is proportional to the intensity of the 
sound affecting the microphone ; and 
(ii) That the amplitude of the oscillatory resistance change oR 2 is proportional to 
the amplitude of the sound which produces it. 
As pointed out at the end of § 6, neither of these conclusions would be expected to 
hold quite exactly in the case of very loud sounds. 
(i) First Experiment .—The object of the experiment was to find out if the change of 
resistance ARj is proportional to the intensity of the sound. In order to do this it is 
only necessary to expose a microphone to different sounds of known relative intensities, 
and to observe in each case the value of ARi- The method adopted in the experiment 
was to observe the effect produced on the microphone when it was placed at various 
distances from a source of sound working at a constant rate, the relative intensities of the 
sound to which the microphone was exposed being deduced from the Inverse Square 
law. 
The source of sound was an electrically maintained tuning-fork vibrating in front of a 
glass-bottle resonator, the frequency of the fork being 250 vibrations per second. The 
fork with its resonator was placed on the ground in a suitable open space.* The 
amplitude of vibration of the fork could be observed by means of a microscope and 
micrometer eyepiece, and it was found that with care the fork could be made to vibrate 
with an amplitude which would remain constant within a few per cent, for quite long 
periods of time. 
The microphone was clamped with its axis vertical in a heavy retort-stand, so that 
it v r as held at a height of about 1 foot 6 inches above the ground. The grid was 
connected by a long pair of leads to a Wheatstone's Bridge, which was set up inside a 
laboratory. A reflecting galvanometer was used, and a preliminary experiment showed 
that for the small changes in resistance to be observed (not exceeding 0 -25 ohm), the 
deflection shown by the galvanometer was proportional to the resistance change in the 
microphone. 
The fork having been set in vibration, the stand carrying the microphone was placed 
at a convenient distance and the reading of the galvanometer noted. A piece of card was 
then placed over the mouth of the glass-bottle resonator, so that the sound from the 
fork became negligible. This enables the observer to obtain a zero reading on the 
galvanometer, the difference between the tw r o readings being the deflection due to the 
sound. The microphone is then moved into another position at a greater or less distance 
from the fork and the process repeated. The result of one experiment is given below. 
The distances vary from 12 to 64 feet, and the deflections shown in the table are the 
means of three or four observations in each position. The actual readings for any 
particular distance did not differ amongst themselves by more than 0-3 cm., 
* The experiment was carried out on Woolwich Common, about one hour after sunset on a calm 
evemng. 
