The slinky, invented ca. 1950, is an excellent device for demonstrating longitudinal waves. How did the demonstrator of the 19th century show these waves?
Ritchie of Boston solved the problem with the wave machine illustrated above, sold in 1860 for $25.00. I use this machine every year for demonstrations at Kenyon College, and find the students are fascinated by the way that the compressions and rarefactions move to the right or left, depending on the direction in which I turn the crank. The design is by Ebenezer Snell of Amherst College.
| Here you can see the mechanism of the Ritchie longitudinal wave machine. The uprights on which the spheres are mounted rest on circular disks set at an angle to the driving shaft. As the shaft and disk revolve, the balls are driven horizontally back in forth in nearly simple harmonic motion. Lots of lubrication makes this machine operate reliably. | 
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The brightly-painted wave machine below is exactly the same as the one
above. However, the large Physics Lecture Halls at the University of Texas
in Austin make it necessary to use bright colors to make demonstrations
visible to the students lurking in the back rows. This is in excellent working
condition and has been in continuous use for about 100 years. 
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The vertical longitudinal wave machine at the left is derived from the Kohl transverse wave machine used for projection. It is in the apparatus collection of the University of Northern Iowa, and the picture was taken by Megan Yasuda of that insitution. The apparatus is subject to the bending of the long, vertical rods, but it is most clever. It can be found in the 1928 catalogue of Max Kohl of Chemnitz, Germany. |
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