50 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



45. Organ pipe-blower. The remaining experiments were made with 

 simple cylindrical tubes, closed at one end with a glass plate or open at both. 

 To evoke a definite note from these tubes, fundamental or overtone, the device 

 shown in figure 73 was designed . This consists of a brass tube P, pinched down 

 at cc', so as to form a crevice 2 or 3 cm. long and not much more than 0.5 mm. 

 broad. From this issues a lamina of air striking the strip of thin brass ss f , 

 about 5 mm. broad. The strip ss', which is always to lie in plane of the lamina, 

 is on guides gg' of thick copper wire, bent at right angles, as shown, and 

 soldered to the ears of the crevice cc'. In proportion as a higher or lower note 

 is to be evoked, ss' is placed nearer cc' or removed from it, or again blown 

 harder or more softly; for the nearer ss' is to cc' the higher the mean pitch of 

 siffling. For high overtones the adjustment i? rather delicate and should be 

 made (preferably) with a micrometer. In the apparatus figure 73, ss' slides 

 with slight friction and is moved by the fingers. In use, the blower is placed 

 across the end of the pipe with the plane gcc'g' normal to the axis. The partic- 

 ular note wanted is obtained by correctly setting ss', which sometimes 

 requires patience. The best results are obtained with pipes of the one-foot 

 octave, and of a diameter less than twice the width cc', pipes of equal width 

 with cc' being most satisfactory. From inch gas-pipe, 2 feet long, a whole 

 series of overtones may be evoked in succession. With a less exacting 

 demand for an immediate response, clear notes may, however, be obtained 



o 



72 



73 



from a great variety of vessels. Thus bottles, deep tumblers and beakers, flat 

 jars (like sardine boxes), truncated cones, thistle-tubes, and even thimbles 

 respond, often very loudly. 



Very disconcerting sounds are otten obtained. Thus, for a wide-mouthed 

 cylindrical jar, 3 inches in diameter and 6 inches high, taperi ig down at the 

 top to a mouth 1.5 inches in diameter, the fundamental appears at once (55' 

 across the middle). If now the distance sc is decreased, the overtone will 

 appear loudly; but it is not the fifth above but the octave itself. As the 

 kinematics of the stationary waves are given, the overtone belongs to an 

 original wave of 1.5 longer wave-length than the fundamental. With a 

 flexible strip ss', like moistened paper, the response is often better as to tone 

 quality and the clarionet note is suggested; but the instrument is less 

 convenient.* 



"Reference should here be made to a more comprehensive apparatus invented by Professor 

 Webster (Proc. Nat. Ac. Sc., July, 1919; Am. Phys. Soc., 1919; Science, Feb. 25, 1919) and 

 adapted for blowing brass instruments. 



