Page:Popular Science Monthly Volume 36.djvu/55

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45
SENSITIVE FLAMES AND SOUND-SHADOWS.

sound to be 1,120 feet, reducing this to inches, and dividing by 1·05 inch, the pitch of the whistle was thus found to be in the neighborhood of thirteen thousand complete vibrations per second. In no other way could this pitch be determined, for the most accomplished musician loses his power of discriminating pitch when either the upper or the lower limit of audition is approached. The pitch of the highest tone employed in music does not exceed five thousand vibrations per second.

PSM V36 D055 Definitions of wavelength.jpg
Fig. 6. — a, advancing sinusoid; b, returning sinusoid; c, advancing and returning sinusoids, forming two loops and a node; c e is a whole wave-length; c d, a half wave-length.

In performing this experiment Lord Rayleigh discovered an interesting peculiarity of the human ear in contrast with the sensitive flame. By using a tube, whose opening was placed alternately in the aërial loops and nodes, and conveying the sound thus to the ear at the same time that the flame was alternately agitated and quiescent, he found that the ear was most affected where the flame was least affected, and vice versa.

The flame, moreover, is unequally sensitive in two directions at right angles with each other. In drilling the small cylindrical hole of the burner no amount of care is sufficient to prevent minute irregularities. The current of issuing gas is not absolutely cylindrical. It is disturbed slightly by interior currents from side to side, and these affect the sensitiveness of the jet to external disturbances. To test this, let the nozzle be rotated on its own axis while the whistle is sounding, until the maximum effect is noticed; and let the sensitiveness of the flame be slightly reduced without causing it to cease to flare. On rotating the nozzle now through a right angle the flame is found to become quiet. Let a mirror be put on one side of the flame, a short distance off, so as to face the sensitive side. Adjusting it until it is equally inclined to the directions of flame and whistle, the flaring is started anew. This ceases when the mirror is rotated toward either side through a very small angle. Indeed, no more beautiful and exact illustration could be devised for showing the law of reflection of sound-waves. The sound-ray, taking a longer and broken path, disturbs the flame on its sensitive side, while the direct rays are at the same time beating in vain against what by analogy we may call its deaf side.

Probably the most interesting acoustic phenomena to be investigated by the aid of the sensitive flame are those of diffrac-