216 
IOWA ACADEMY OF SCIENCE Voi.. XXVII, 1920 
Fig. 45. Iv = Auto-transformer for feed back. Li and L 2 = Receiver coils. 
C =: Tuning capacity. Ci = Large capacity (2 m. f.) as path for high frequencies 
to avoid 15 V. battery. 
the magnetic fields owing to this current oppose each other, 
and produce a radial field in the aluminum diaphram. 
The alternating current produces a magnetic field which is axial 
and hence induces circular currents in the diaphragm which react 
with the radial field due to the direct current producing vibration 
of the diaphragm of frequency equal to that of the alternating 
current. The air spaces between the concentric cells forming 
the pancake coils are for the purpose of allowing the sound waves 
to leave the diaphragm. With this arrangement sounds from 5000 
to 26000 vibrations per second have been produced. With inten- 
sity the sound may be altered by changing the value of the direct 
current flowing through the coils. With this apparatus a beam 
of sound may be reflected in any direction by a mirror made of a 
flat piece of board about a foot square, 
Mr. C. E. Lane, a senior in this university, has worked with me 
throughout this piece of work and has been of much assistance 
with valuable suggestions and efficient and careful experimenting. 
Physical Laboratory, 
State University of Iowa. 
