ATMOSPHERE AND THE ACOUSTIC EFFICIENCY OF FOG-SIGNAL MACHINERY. 233 
telescoping cylindrical Helmholtz resonator capable of adjustment to any required 
pitch by suitably adjusting its volume. Over a circular aperture at the back of this 
resonator is clamped a mica disc, at the centre of which is firmly fastened a brass pin 
carefully ground to a fine point. At right angles to this pin, held in a fork carried 
by the frame of the resonator and capable of fine screw adjustments in two directions 
at right angles, is mounted a thin steel strip which may be so adjusted that the 
point of the pin above referred to rests in contact with the strip at any required 
distance from the neutral axis. A slight displacement of the disc thus causes the 
strip to rotate through a small angle. This rotation is magnified into an easily 
Fig. 2. The Webster phonometer. 
Designed by Prof. Webster for the Father Point tests. 
measurable deflection by viewing the light from the filament of a small tungsten 
lamp, reflected from a small concave mirror attached to the steel strip, into a 
micrometer eye-piece fitted with a suitable graduated transparent scale. When the 
diaphragm is at rest, the image of the filament is seen sharply defined against the 
micrometer scale ; when a sound-wave of the proper pitch passes over the instrument 
forced oscillations are set up in the air contained in the resonator. The diaphragm 
(whose free period is much higher than that of the sound-wave to be measured) is 
thus set into vibration and the image of the filament is drawn out into a luminous 
band whose breadth may easily be measured to an accuracy of about 1 per cent. 
