1021] on Absolute Measurements of Sound 409 



should not be such as to make the representative point occur at the 

 middle of the figure, making both mistunings zero, but that both 

 mistunings should be of the same sign and a certain magnitude, 

 depending on the coefficients of damping of the two degrees of 

 freedom of the coupled system. The mathematical theory is precisely 

 that of a wireless receiver. The ultimate sensitiveness depends on 

 the smallness of the damping of the plate. 



The apparatus as it was built several years ago was mounted upon 

 a heavy bronze stand, covered at the back by a heavy bronze cover 

 to keep out the sound, while the three shafts turning the screws of 

 the interferometer adjustment protruded through sound-tight fittings. 

 Upon the front of the instrument a properly tuned resonator was 

 attached, and at the side was a small incandescent lamp with a 

 straight, horizontal filament, an image of which was projected by a 

 lens upon the first mirror of the interferometer. Upon this was 

 focused a telescope, giving in the reticule an image of the horizontal, 

 straight filament, crossed by the vertical interference fringes seen 

 with white light. In order to get these the plate must be in the 

 proper position within a few hundred-thousandths of an inch. The 

 objective of the tuning-fork was carried by a tuning-fork which 

 oscillated vertically, tuned to the pitch of the pure tone to be examined, 

 and this, combined with the horizontal motion of the fringes, resulted 

 in a figure of coloured fringes in the form of an ellipse. On slightly 

 mistiming the fork, the ellipse could be made to go through all its 

 phases, and when it was reduced to an inclined straight line its 

 inclination was read off on a tangent scale. The amplitude of the 

 compression of the air in the sound was then directly proportional 

 to the scale-reading. 



"While the interferometer is still used for calibration, the movement 

 of the diaphragm is recorded for actual measurements by a thin steel 

 torsion strip carrying a concave mirror. A lamp with a vertical, 

 straight filament is viewed through a telescope into which the small 

 mirror focuses the image of the filament on the reticule, and a 

 magnification of from 1200 to 1500 is used, so that the sensitiveness 

 is about the same as with the interferometer. 



At first the only method of tuning was the clumsy one of changing 

 the mass of the diaphragm by adding small pieces of wax. This was 

 not capable of continuous variation. Now the diaphragm has been 

 discarded and replaced by a rigid disc supported by three steel wires 

 in tension. The disc is made of mica or aluminium, and is carried 

 by a little steel spider containing three clamps to hold the wire. The 

 tension is regulated by three steel pegs, one of which is controlled by 

 a micrometer screw. The disc is placed in the circular hole through 

 which the sound enters the resonator. This has the advantage of 

 reducing damping very largely, and thus of increasing the sensitiveness 

 enormously. The instrument now competes with the human ear, and 

 can be tuned over two octaves or more. 

 Vol. XXIII. (Xo. 115) 2 f 



