Frequency Indicator
The "Frahm-System" Frequency Indicator below, shown open and closed, was given to the Greenslade Collection by Daniel Chaucer. A series of thirty-one tuned reeds in each of the two banks are driven by an alternating current signal whose voltage may be chosen by making the proper connections to the connectors across the top. The lower bank runs from 52.5 Hz to 67.5 Hz in 0.5 Hz steps, and the lower bank runs from 21 to 28.5 Hz in 0.25 Hz steps. The case appears to be made of walnut. The apparatus bears the Logo of Siemens-Halske of Berlin and was imported by Biddle of Philadelphia.


 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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