You cause a metal rod to vibrate in your fingers and are rewarded when it sings to you.
This classic science demonstration is sure to wake you up… and probably the people down the street… and for sure every dog in the neighborhood! It’s easy enough: hold an aluminum rod and simply slide rosined fingers along it lightly to produce ethereal sounds. It takes some patience but with a little practice and some science know-how, you’ll become a Singing Rod virtuoso.
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- Solid aluminum rod, 5/8" (16 mm) diameter and 24" (61 cm) long
- Violin rosin
- Patience and practice
- Adult supervision
Make sure the rod is clean and straight. Hold it lightly at the center balance point with your non-writing hand. Place your first and second fingers on top with a thumb supporting it underneath. The key is to have as little contact with the rod as possible. That increases the likelihood of vibrations.
Apply some rosin to the thumb and fingers of your writing hand and lightly coat the rod with rosin.
Gently pinch the rod while sliding your rosin-coated fingers smoothly from the center point to one end of the rod. As soon as you reach the end of the rod, repeat the pinch-n-slide process… again… and again… and again. It may be upwards of 20 times before the metal bar begins to resonate with a sound you can hear. Just don’t give up.
Because of the rosin, your fingers will stick and slide over the bar causing it to vibrate.
The sound you hear will likely be soft at first but the volume increases with each successive slide of your fingers. Remember that each slide reinforces the vibrations of the previous slide. Pinch and slide, pinch and slide, pinch and slide…don’t give up! The resulting high pitch sound can be ear-splitting!
How Does It Work
If you can hear something, that means vibrations are causing sound waves to travel through matter (solid, liquid, or gas) to your ears. If there are no vibrations, there is no sound to detect.
Rosin is used to increase friction between your fingers and the rod. It causes your fingers to stick and slide as they move along the bar. This repeated sticking and sliding action basically causes your fingers to tap the bar and set up vibrations in it. The space between sticking and sliding is so tiny that you probably won’t feel it happening. When you hear a tone, however, you’ll know it’s happening.
You probably noticed that tapping the bar on the sides produced a lower pitch while striking the bar on an end produced a higher-pitched sound. The same high-pitched sound is made by sliding your fingers on the bar. In both cases, the higher pitch resulted from the formation of compression (or longitudinal) waves in the bar. Each successive stroke of your fingers on the bar reinforces the strength of the previously established compression wave, resulting in a louder and louder sound.
To model a compression wave, picture a Slinky toy stretched out on the floor between two people. One person grabs several coils of the spring, pulls them straight back to the end of the toy (compresses them), and lets go. The energy from the compressed coils moves quickly through the entire length of the spring from one end to the other and back. The high pitch sound of the metal rod is the result of a compression wave traveling through the entire length of the bar.
However, if the rod is held in the middle but tapped on a side with a solid object, a transverse wave is created at that point. These waves have longer wavelengths (crest to crest) and, as a result, have a lower pitch than compression waves. A transverse wave is modeled by quickly moving the Slinky in a left-right motion on the floor. This movement creates nodes and antinodes as well as the formation of standing waves (a series of figure-8s) along the toy.
Practice hitting the bar with a solid object close to the end with a slightly diagonal stroke. By doing so, you actually create both compression and transverse waves at the same time. You’ll be able to hear both sounds these waves make at once, too.
Take It Further
Test These Ideas:
- Use different lengths of rod to produce tones the same way
- You could also test different kinds of metal to see if/how the tones change.
- Hold the bar in the middle and tap the sides. Why did the sound seem different when you tapped it instead of sliding over it?
- Compare this to the sound produced by hitting the bar directly on an end. How does this sound compare to the sound made by stroking the bar?
- Why does the metal bar vibrate when you rub it with your fingers? Why is the rosin necessary
- Where is the high-pitched sound coming from (middle, sides, or ends of the bar) and why?
Over the years, this activity has been published in a number of science demonstration books. My initial efforts to master the Singing Rod always resulted in tired fingers and little to no sound.
Success finally came after seeing Dr. Albert Baez present the technique several years ago. The motion of repeatedly sliding your fingers over the bar needs to be smooth and graceful. Dr. Baez effortlessly brought tones from a metal bar that produced high-pitched sounds that filled an auditorium. It was amazing. You can do it, too!