400 
DR. W. S. TUCKER AM) MR. E. T. PARIS OK 
As a rough check on these observations, we may calculate from hydrodynamics! 
principles approximate upper and lower limits to the conductivity of the neck. The 
required expression is given by Rayleigh (“ Theory of Sound,” vol. II., p. 181). For 
a cylindrical neck of length L and radius R, 
_ 7rR 2 
'' “ L + all ’ 
where aR is the “ end correction ” to be added to L on account of both ends. Since 
one end is flanged and the other unflanged, we take a — 0 *8 + 0 -6 = 1 -4. To find 
an upper limit to c, take L = 2 -2 cms., 2R = -0 -75 cm. ; and for the lower limit take 
L = 2 -2 cms., 2R = 0 -65 cm. (the diameter of the circular hole in the mica plate). 
We then find 
0-162 > c > 0-125. 
§ 5. Sensitivity. 
The following experiment gives some idea of the smallness of the sound of which the 
microphone is capable of recording when it is used in conjunction with an amplifier. 
A microphone was constructed from brass-tubing 1 inch in diameter and tuned to 
respond to a note of 256 vibrations per second. This microphone was placed in one corner 
of a large field and connected by a long pair of leads to an army amplifier of the 
“ C Mark II.” pattern, the output terminals of which could be connected either to a 
pair of Brown telephones or to a Campbell vibration-galvanometer (also tuned to 256 
vibrations per second). In order to test the sensitivity of the microphone, a tuning-fork 
giving a note of frequency 256 was sounded over a resonator at various distances from 
the microphone. The sound produced was as a rule inaudible to the unaided ear at 
distances greater than 80 yards. It could, however, be heard in the telephones up 
to distances of about 200 yards, and when the vibration-galvanometer was used quite 
well-marked deflections were obtained up to a distance of 400 yards or more. 
As regards the conditions which determine the sensitivity of the microphone, these 
may be divided into two groups according to whether they have reference to the resonator 
or to the grid. We will deal first with those that refer to the resonator. Amongst 
them the most important is obviously that the resonator be accurately tuned to the note 
which it is required to record. The effect of mistiming is clearly shown by the resonance 
curve in fig. 5. 
When the resonator is accurately tuned, i.e., when n = p, the maximum velocity 
in the neck, according to the equations given in § 4, is 
