Davis — Anemometer for Stationary Sound Waves. 129 



Art. X. — On a Miniature Anemometer for Stationary 

 Sound Waves ; by Bergen Davis, Ph.D. 



The anemometer which is in such general use for measur- 

 ing wind velocities possesses the convenient property of rotat- 

 ing in the same direction irrespective of the direction of the 

 air currents acting upon it. 



In a stationary sound wave we have the condition of an 

 alternating current or impulse of air, the direction of whose 

 motion is twice reversed during each complete vibration. Such 

 alternating currents of air, if of sufficient magnitude, should 

 act upon an anemometer in the same manner as a continuous 

 current, provided the cups of the anemometer are sufficiently 

 small. The writer has constructed a number of miniature 

 anemometers which have beeu found to rotate with consider- 

 able velocity in a stationary sound wave. The rate of rotation 

 varied with the position of the anemometer along the wave. 

 It was a maximum at the middle of the loop, and the rotation 

 ceased entirely at the nodes. 



An investigation of this effect was undertaken for the pur- 

 pose of determining the relation between the amplitude of 

 vibration and the rate of rotation, and also the influence of the 

 size of the anemometer cups. 



The stationary wave used was that produced in a stopped 

 organ-pipe speaking its first overtone. Two nodes were thus 

 obtained, one at the stopped end and the other a short distance 

 from the mouth. A thin rubber diaphragm was placed across 

 the pipe at the node nearest the mouth for the purpose of pro- 

 tecting the interior of the pipe from disturbances that might 

 arise from blowing. The portion of the pipe between the 

 diaphragm and the stopped end contained one-half of the sta- 

 tionary wave. The side of the pipe from the diaphragm to 

 the stopped end was removed and a glass panel substituted for 

 it, thus rendering the interior of the pipe visible. The pipe 

 was 68 cm long from lip to stop, and 5*3 by 6*4 cm in cross-sec- 

 tion. The diaphragm was placed 16 cm from the lip, as this was 

 found to be the position of the first node. The half wave 

 between the diaphragm and the stop was 52 cm long. A centi- 

 meter scale was placed along the pipe with the zero at the 

 stopped end for the purpose of measuring distances along the 

 wave. 



The cups of the anemometers were made from gelatine cap- 

 sules such as are used for medical purposes. These capsules 

 were divided longitudinally, each part so obtained forming a 



Am. Jour. Sci.— Fourth Series, Yol. XIII, No. 1 4.— February, 1902. 

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