The Ringing Bar
  This is a classic example of a piece of apparatus which can be used for two different demonstrations. In its original form, it was intended to demonstrate longitudinal waves. The left end of the bar was set into oscillation by drawing a rosined cloth down its length. The brass bar is clamped at the center, forcing a node at this point; the two ends are thus antinodes. The ivory ball, hung by two cords, rests lightly against the right-hand end of the bar, and flies out repeatedly due to the oscillating force exerted on it by the longitudinal vibrations of the bar.
   In the late twentieth century I have used this apparatus at Kenyon College to demonstrate and measure the speed of sound in brass. The ivory ball bounces against the end of the rod, producing a clear, ringing sound. Beats between this sound and that produced by a loudspeaker driven by a calibrated function generator are used to measure the frequency of the sound. The wavelength of the sound in the bar is twice the length of the bar. For this piece of apparatus, the speed of sound is 3576 m/s, typical of brass.

   A similar apparatus in the Catalogue of Acoustic Apparatus published in 1889 by Rudolph Koenig of Paris cost 45 francs.

REFERENCE: Thomas B. Greenslade, Jr., "Nineteenth Century Textbook Illustrations XXVIII: The Ringing Bar", Phys. Teach., 17, 44 (1979)

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