The Frahm Frequency Indicator at the left was distributed by James G. Biddle of Philadelphia, and is in the collection of Kenyon College in Gambier, Ohio.
This indicator has a wider range than the one above, as its response ranges from 20 to 80 Hz .
This mechanical resonance device uses the Frahm principle, in which metal
reeds are driven at their resonant frequencies. The heavy wheel has a small
stud in it that causes the wheel to rotate with a slight side-to-side wobble.
The wheel is started spinning like a top, and as it slows down, the rotation
frequency matches the resonant frequency of the shortest reed. Further slowing
of the rotation rate causes increasingly longer reeds to vibrate. This device
was sold by the Chicago Apparatus Company for $18.50 in the second quarter
of the 20th century, and is in the apparatus collection of Antioch University
in Yellow Springs, Ohio.
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