Today, let’s take look at a popular multi-function demonstration apparatus: The Shive Wave Machine. You can see it in action in this new video featuring Prof. Peter Shawhan.
Invented in 1959 by Dr. John Shive, a Baltimore native who became a physicist at Bell Labs, the Shive Wave Machine consists of a series of heavy steel rods connected at their centers by a stiff wire; it functions like a torsional spring. You can create a pulse by gently moving the rod at one end up and down; then you can see how the pulse propagates as a wave along the length of the wire, watching each rod move in turn. Our demonstration collection has two such devices; one has longer rods, the other has shorter rods; as you can see in the video, the different weights thus give them different transmission speeds and impedances.
This can be used to demonstrate a variety of wave phenomena. You can make a simple model of a traveling wave by first sending a single pulse down the length of the apparatus. The torsion wire in the center transmits energy from one rod to the next. As in all mechanical waves, the individual components that make up the wave, whether they be rods on a wire or molecules in air or water, do not travel far from their starting points; what travels from the beginning to end is not the particles or other elements, but the energy of the wave and the pattern of disturbance it creates. The disturbance moves down the line, but the rods return to their starting points. No matter how large or small the pulse, it travels at the same speed within a given medium, determined by the impedance. If we create the same pulse on the other unit, with a different impedance, the wave travels at a different speed.
Similarly, you can model superposition of waves by sending pulses from both ends simultaneously. We can see that the two pulses pass through one another without interacting. At the point where they cross, the pulses may superimpose additively or subtractively, depending on whether they are in phase or out of phase, but they then each move on in their original directions.
When we send a pulse down the length of the device and it reaches the far end, the pulse reflects off the open end and returns to where it started, looking much like it did when it was going the other way.
But if we clamp the far end in place so that the last rod is fixed in place, then when we send the same pulse down the length of the device and it reaches the end, it still reflects back – but inverted! The pulse is upside-down from its original orientation. Whether reflection from an end is upright or inverted depends on whether the end is open and free to move, or closed and fixed. If instead we clamp the two Shive devices with different impedances together, we can see a partial reflection – part of the pulse reflects back and part of it passes on into the second medium. How much reflects and how much passes through depends on the difference in impedance. The reflection from an open or fixed end is essentially the extreme case of this – in a sense, it’s reflecting back all of the energy from something with effectively zero or infinite impedance.
You can see Prof. Shawhan put the Shive apparatus through its paces in the video above! To explore this device some more, try out this simulator [click here for simulator]. This simulator replicates some of the behaviours of the Shive wave machine. The dropdown menu at the top lets to adjust the configuration, and the Play button can initiate a pulse.
Also, you can make a simple one at home! The Science House at North Carolina State University has published plans for making your own simple wave machine from household items. Check them out in pdf form here: https://sciencehouse.ncsu.edu/wp-content/uploads/2017/03/Soda-Straw-Torsional-Waves-Oscillations.pdf and visit their site for other fun remote learning ideas!
Some Shive Wave Machine demonstrations in our collection
- G3-01: Traveling Waves
- G3-02: Superposition of Pulses
- G3-03: Reflection of Pulses
- G3-04: Standing Waves
- G3-05: Partial Reflections
- G3-06: Impedance Matching
- G3-07: Tapered Transformer