Longitudinal wave demo with suspended slinky.
What it shows:
Demonstration of longitudinal traveling waves in a spring.
How it works:
The Slinky hangs with a bifilar suspension from a rigid thin-walled electrical conduit frame, which is light, strong and cheap. In total, 23 suspension points run the length of the spring; the cord is a thick cotton thread that attaches to a loop of the Slinky with No.10 fishing swivels. The layout of the Slinky and frame are shown in figure 1, but the thread has been omitted for clarity.
figure 1. Slinky Wave Cradle
At one end of the cradle is an aluminum plate with a central hole the diameter of the Slinky. Through this hole the Slinky is attached to a rubber membrane that is a fragment of a tire inner tube. The inner tube valve can be used as a knob, to be pulled back and released, sending a pulse down the Slinky (sort of like a pinball machine).
Setting it up:
The frame sits comfortably on a lecture bench. To add to its visibility, the Slinky has been sprayed with luminous orange paint. Mount a 4ft UV light before it and demonstrate with the lights down.
Generating good longitudinal waves has always been trickier than good transverse waves. Sending pulses down a Slinky that lies limply on the bench is pretty awful, and this wave cradle is an excellent solution. The bifilar suspension cuts out effectively all transverse waves, and with an optimum number of supports, the spring is perfectly horizontal.