A Simple Mechanical Resonance Demonstrator




Introduction: A Simple Mechanical Resonance Demonstrator

"In physics, resonance is the tendency of a system to oscillate at maximum amplitude at a certain frequency. This frequency is known as the system's resonance frequency. When damping is small, the resonance frequency is approximately equal to the natural frequency of the system, which is the frequency of free vibrations." ( from Wikipedia, 8/8/2007)

With this Instructable I'll show you a simple rig that can be used to experiment with electromechanical resonance.

Step 1: Forewords

Of all the aspects of physics we can investigate with simple household items, magnetism is the most exciting to me. Although I'm sort of grown up I still love pushing magnets one against the other just to feel the magic repulsive force.
Repulsive force for some reason is more entertaining than the attractive one. In this field, I mean.

We know that when a current flows through a metallic wire a magnetic field is generated; that's what electric motor are based upon.
The wire can be wound in the shape of a bobbing to make the field stronger and a metallic core can increase the strength of the field. The equations linking number of turns, coil diameter and current can be found here (http://en.wikipedia.org/wiki/Electromagnet) but it is not strictly necessary to understand the underlying principles.
A direct current, the one from batteries, generates a constant magnetic field. The direction of the magnetic field with respect to current is established conventionally by the so-called right hand rule : with reference to the picture, grabbing the coil with our right hand with the thumb pointing the same direction of the current, the magnetic field will have the direction of the other four fingers.
The direction of current is, again, conventionally fixed going from the + pole of the battery through the wire to the - pole.
Alternating current (a.c.), the one that comes out a regular wall socket, will have the same effect except that the intensity of the current flowing through the coil wire cyclically rises to a maximum then decreases to a minimum in the opposite direction passing through a zero current level. As a consequence, a.c. powering a coil generates an alternating magnetic field that cyclically rises to a maximum value then decreases to a minimum passing through a zero field value.
It is important to note that the field does not rotate in any direction. Simply rises and falls in intensity then reverse its verse and rises again.

Step 2: Two Is Better Than One (and Three Would Be a Crowd)

I used two coils facing one another. This arrangement forms a sort of Helmholtz coil ( http://en.wikipedia.org/wiki/Helmholtz_coil ). A sort of, because the exact arrangement requires the two coils to be at a distance equal to the radius of the coils. I had to make the distance bigger than that because I wouldn't have had enough room to experiment.
With these two coils a region of reasonably uniform magnetic field (B field) is obtained between the coils.
The two coils are connected in series so as that the two windings have the same winding verse. I couldn't visually check the verse so I had to experiment. Later on on this point.
The two coils are powered by a wall a.c. transformer. Extremely important: do not try to power the rig directly from the mains: it doesn't work, it would destroy the coils and possibly cause burning and electric hazard. An a.c. low voltage output transformer is mandatory.
Also, make sure that the output of the wall transformer is not direct current (d.c.) as it won't work for us.
In case you can't find a wall a.c. output transformer you can build one yourself. In case, it shouldn't be difficult for a friend to help you.

Step 3: Construction

A few parts are necessary: most of them are common household items while a few can be bought or scavenged.
1. The coils I took from a small stepper motor. I demolished the one I used here a while ago so I haven't any picture. Anyways, a small/medium size stepper motor can be taken off a printer. Larger ones have a different internal structure so no useful coil can be taken from them. In case, a bobbin could be made out of thin cardboard or plastic covered with 200 turns of magnet insulated copper wire (Amazon p.n. B000IJW7YU should do)
2. The wall adapter I used was part of a dead DECT wireless telephone. For some reason it supplies d.c. and a.c. (on different pins of its plug). The output is 6Vac, different values are just fine.
3. Rare earth magnets: essential part of any modern household with kids they can be bought easily. May I suggest the original Plastwood (r) Supermag (r) ? they are proudly made in Sardegna ( http://www.plastwood.com/en/ ). Do not settle for substitutes ! They can be bought everywhere where good things are sold like Amazon ( e.g. p.n. B000TGU7FS ) for as low as 13 USD. The more you have in your home the more can be ingested by small kids...be careful.
4. Rubber bands.
5. Insulated wire, both for holding the coils in place and for electric wiring.
6. Small chunk of scrap wood 8"x8". A hard cover book can be used instead. You could buy it from Amazon...there's a plenty in there to choose from ! I was 'bout to use a Harry Potter I never read.
In retrospect I should have done it : the magic of magnetism and the magic of Potter really match.
7. Safety goggles (Amazon.com p.n. B000F712UA, just to point to one of the many you can find there, should do).

First you need to mark the wood board and drill the holes to keep the coils in place as in the picture.
You'll then need to hold the coils in place with some insulated wire and connect the two coils in series. Try to connect the two coils so as the two windings have the same verse; should it be hard to find the verse connect then disregarding the verse. Connect the power the two coils and keep the magnet between the two coils you should feel a strong vibration. If not, reverse one of the two windings.
Tie the magnet to a rubber band. The North and South poles of the magnet should be along the direction of the two coils.

Step 4: Showtime !

Now watch the video, you may have already done so at Intro. It shows also how to operate the rig. I tried to add captions to it but I wasn't able...
Anyways, you can see that after attaching the power the rubber band with the magnet is placed between the coils. Pulling or releasing the rubber band rises or lowers the natural frequency of oscillations of the system rubber band+magnet. When the natural frequency equals the one of the mains, 50 Hz or 60 Hz depending on where you live, the oscillations become very smooth. When the natural frequency is off the mains' the oscillation is very irregular.
When the oscillation is smooth the systems is said to be in natural forced oscillation and the maximum energy is transferred to the system. In the video oscillations don't look as smooth as they are in reality.

You may have watched the footage regarding the Tacoma bridge falling a while ago. In that case the wind made the air vibrate closer and closer to the natural frequency of oscillation of the bridge till a great deal of energy was transferred to the bridge making it collapse.

Less catastrophically, sure you have pushed a swing: it is easier to push a swing at its natural frequency (the one we experiment when in free oscillation) than to force it to oscillate at any other user-defined frequency.

Please wear your safety goggles when operating the thing.

One note: after connecting the power to the circuit check the coils for heat: if they feel too hot, disconnect the power and go for a lower voltage transformer or coils with more turns. Sorry, otherwise the fun wouldn't last long.

Again : do not operate this rig directly from the mains, it won't work and very likely you would burn the coils, ignite something and cause hazard.

Step 5: Final Considerations and Hints for Further Investigations

I had built a similar thing when I was a student. It was based on larger bobbins and stronger current than I'm using now and the effect was really amazing. Later, I disposed of everything while moving to a new home. Too bad I could now replicate only a smaller copy of it, but the effect is still noticeable.

A regular electric razor is one example of device whose efficiency is based on a resonance.
I opened an old Braun razor and cleaned it. Amazing how dirty can be the INSIDE of a razor, almost as a TV set.
From the pictures you can see the structure of a razor: two coils generate an alternate magnetic field. Between its polar expansions (the metallic U-shaped core inside the two coils) a magnet with spring is placed. The a.c. generates an alternating magnetic field which in turn forces the magnet to oscillate and swing the razor blades.
Now the interesting part: the red paint dot keeps a tuning screw locked. The screw is necessary to tune at the factory the natural resonant frequency of the system magnet+spring and match it to the mains' so as to transfer the maximum energy to the spring+magnet (and blades) and give the razor a more natural handling feel and regular blade travel.

I dismantled the razor and tried to use the bobbins and polar expansions for my circuit but the position of the expansions is not very suitable.

Be carefully if playing with and open razors. Electric shock is always awaiting for careless curious people and metallic part could spring out of the oscillating bridge. Wear your safety goggles. Don't operate and open razor unless you know exactly what you are doing. Always have someone around you whom know what to do in case.
Also, do not be tempted into touching the razor's coils when the razor is powered.

A further interesting experiment with my circuit could be to place a low value resistor (1 Ohm or less) in series with the coils and check the current flowing through it with an oscilloscope and see what happens to the current in and out of resonance.

Ciao e buon divertimento (have fun !)



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    40 Discussions

    How many turns should the two bobbins have?

    I remember i saw, once, a device who was using this property. The oscillating magnet was used (i don't remember exactly how) to produce current into a third coil. And the energy generated into the third coild was slightly greater than the energy supplied to the system ... giving, thus, a device with an efficiency slightly greater than 1. Though, chances are great that it was a pure hoax ...

    7 replies

    lol. Are your serious? When did the world stop turning????? And dont even get me started on the macroscopic....

    In fact, the Earth's rotation is indeed slowing down. Google "earth rotation slowing". You'll get 111,000 results, like this:


    My experience with perpetual motion is that it's impossible unless it somehow involves women. (Ducking sudden appearance of hurled frying pans and irons).


    No, it is real. No it is not perpetual motion. The device is called the Vacuum Triode Amplifier, or VTA. Some also know it by its name previous to that, the Space Quantum Modulator, or SQM. The inventor, Floyd Sweet, used a special conditioning process to coax barium ferrite magnets into self-oscillation, at which point a small driver current was fed into a coil parallel to the oscillating magnetic field, and another output coil was placed perpendicular to them and yes, the deivice was capable of achieving overunity (COP/efficiency>1). However, the electricity it put out was unlike any other. It was a 'cold' electricity, and the device in question would run twenty degrees celsius BELOW ambient temperature. Hope this helps. Cheers.

    It's frustrating. I'm still unable to find it ... I remember it was related to diode or transistors (a kind of "mechanical" transistor maybe ?) ... Tsss ... that's internet ... when you're not looking for something special, you find it. And when you're looking for something special, you can't find it .....

    Technically, that could be. Though, it was not what the device i was talking about was used for ... Damn, I don't remember the name of this device neither the name of the inventor ...

    Could you turn a DC adapter into an AC one by removing the rectifier? I haven't had occasion to pop open a wall wart--I don't know how the rectifiers are attached.

    2 replies

    You can as long as it's not a SWITCHING power supply. An AC supply requires a transformer. These are quite heavy for the amount of current they're rated at. All of my 1 amp transformer based supplies weigh in around a good half pound, and get pretty warm in use. My 1 amp switchers are maybe an ounce or so, and if these get warm, something's usually wrong.

    Switching supplies are usually labelled as such.


    Yes you can, provided that the adapter is based on an 'heavy' iron transformer. Lighter ones (those of most modern and most expensive cell phone ) use special ('switchers') techniques to deliver DC from line. They are not good to the purpose. Inside they look like a regular electronic circuit with small (less than 1" x 1") components.

    The AC in 'heavy' iron transformer based adapters can be taken right at the wires coming out the transformer (should look very similar to this one http://en.wikipedia.org/wiki/Image:Transformer.filament.agr.jpg ) before they go to the rectifier. The rectifier could have the shape of a black tiny cylinder or box or could be made with one, two or four diodes.

    Be careful and stay away from the primary high voltage.
    Always make connections with the adapter off line.
    Always seek help from expert friends.

    If you need more directions just write.

    Ciao e divertiti. (have fun)

    Really cool. I'm taking a physics course dealing with magnetism / circuits and this is a great real-life scenario and application of concepts you see in textbooks. I will definitely try this: experiments always make science more fun. Good job.

    2 replies

    Thank you, just consider that the more current goes through the windings the stronger the field ( as you of course already know ) and stronger the effect. The magnet might be also thrown away if the field and the magnet are very strong, be extremely careful, use safety goggles. Ciao Alex

    Using a strong magnet allows you to "bias" the circuit. Otherwise, both positive and negative half cycles pull nonmagnetized objects, effectively doubling the frequency.

    The same can be done with a DC current flow equal to the maximum peak to peak current of the amp. Ohms law can be used to determine this.


    Interesting project! However, my Windows XP won't play the avi movie file :-(

    1 reply

    It will if you have the proper codec. I tired of looking, and now use Media Player Classic Home Cinema.


    and get all the codec's you'll ever need:



    Thanks for this instructable. I am interested in the tuning you mentioned. I am working on a project using electromagnetic force to vibrate objects for musical purposes. what is the tuner? might i be able to use it to excite different resonant frequencies in a given form ..say a metal bin? i am new to this art form ..having spent some years playing with ceramic material i am now in the middle of metal things and vibrations are forever calling me! any thoughts on how i could play the different partials of an objects with eltromagnets? smile. thanks

    2 replies

    I think you refer to the last step and the razor tuning screw.
    In case of a large bin you may need to to tune the oscillator frequency as the bin is home to a large number of natural frequencies. In case you need a sinusoidal oscillator, a large strong coil with an iron core, a power audio amplifier suitable to drive the coil and a large magnet to be placed somewhere around the bin or at its bottom. Connect the coil to the amplifier (do not connect any speaker - expect that you might destroy your amplifier, so you'd better not use your home Harman Kardon HiHiFi amplifier).
    Turn on the amplifier at a low level first and place the coil near the magnet on the bin then start playing with the frequency of the oscillator to excite the partials of the bin. When you hear something interesting, pull up the volume and enjoy.
    Again, be prepared to damage your amplifier, wear safety goggles as the magnet might be ejected.

    To protect expensive (or even cheap) power amplifiers, for solid state, use power resistors in series to insure the minimum load the amplifier is specified for, i.e. 2 ohms. Shorting a solid state amplifier will fry it.

    If it's a tube amp, the opposite is true. You must NEVER operate a tube power amp with no load. It will generate high voltages that will arc the power tubes and output transformer. Tube power amps will withstand output short circuits that would quickly toast a solid state power amp.