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Tesla's earthquake machine - was it just a practical joke? Answered

Nicola tesla, in his early years, was not shy when it came to use science through direct contact.
Most of it we would call today playing practical jokes on students and friends alike.

His harmonic balancer got the nick name earthquake machine.
I won't go into the story how all came together, you can read the story on Wikipedia or other sites to the fullest of your imagination.
Howver, I would like to go into some details of this invention.
Or better: I will try to explain my view on why this machine might have actually worked close to what the story tells....

Studying old paents and other documents is no fun and not always you can reach any valid conclusions from any of it.
I mean, if you coud then you wouldn't need a patent for it.
Some things however are so good, so special that they never get a patent, the company keeps it as an iternal secret.
Same way a brewery won't tell you the exact recipe for the beer they make ;)
The harmonic balancer has very little to go with except stories and some comments Tesla scattered here and there.
So let's start by some claims Tesla made about this machine:
1. It has a free swinging mass.
2. The mass is balanced using "air springs".
3. It usues very little energy.
4. It finds the harmonic frequency of the object attached to automatically.

If you think about the above you could say right away it is bogus.
But think about the terms and language of the old days ;)
Add the fact that Tesla never really gave any direct answers to how his inventions actually work and you see where I am going here.
Point 1 and 3 are easy to imagine in many ways.
Number 2 on the other hand contradicts itself at a first glance.
But, if you place a piston with a good mass and good seal into a cylinder nd close both ends while the piston is in the center....
A bit like these to shake emergency torches and flashlights.
With that idea in the back of a head one can imagine shock absorbers and more.
And a "mass" "swinging" in such a sealed tube would certainly be subject to the "spring" force of the air being compressed in front and the forming low pressure on the other end.
The last one however had me stumped for almost two years with me getting nowhere.
A mechanical system can't adjust itself to the harmonic frequency of anything....
Then I saw a collection of funny videos.
One of the clips had about 100 metronomes standing on a board.
The board was place on two rollers so it cold move freely from side to side.
All the metronomes wre set to about the same timing.
But of course you just can't start them all at once or even so they swing in sync.
After just a short while though, one by one found a matching partner on the board.
Faster ones slowed down, ones totally out of sync seemed to miss a beat here and there.
And then it hit me!
Hundret swinging masses on a board and the board moves together with them!
Once all were in sync and harmony the board moved basically exactly the same way sidewas as the single pendulums above.
Number four finally solved :)

How though could one try to replicate this impossible invention these days and without knowing any exact details anyway?
We know it used one or two electromagnets.
These moved the mass back and forth horizontally in the machine.
However, it was more formed like a T wih a fat base.
Tesla needed a dead simple way of doing with electricity what the metronome does just mechanically.
If you have a little mass on a spring and one contact on the spring plus another on a tube that is around the mass - what do you get?
No, not the trigger for something really nasty when you pick it up...
Right! You get a motion detecting switch.
A bit like our gyroscopic sensors do now in our phones....
And if you cut the tube lenghtwise you end with two opposing contact to switch two magnets on depending on what side of the tube is hit by the mass (contact) swining on the spring.
Try it out with a spring from old pen and a screw inside as a weight - quite sensitive if you get the weight right.
Some drawing suggest he height was about the same as the width of the machine.
This would suggest the contact was actually hanging upside down, so it could swing in the wider base.
A bit of fine and very flexible opper braid with a weight would make a nice pendulum here....
Do a little experiment first though ;)
Make a sturdy frame or if you have none try a truck. ;)
If you place the frame on wheels and a pendulum with just a string and some sinker at the end - what would happen if you move th frame sideways instead of swinging the weight?
Inertia keeps the weight in place while the top mount of the string moves away.
And if the weight stays in place for just a fraction of a second before starting to move with the frame?
Correct! since the angle of the string changes, the ball appeas to move up.
Not really by much but enough for an electrical contact to open....
We have some ideas now on how those electromagnets might have been triggered by Tesla, what about the mechanics of the moving mass that claimed to have created earthquake like results?

Precision is no problem these days, just check these toy kits to build your own Stirling engine or you trusty old Swiss Army Knife.
And even a second hand air cyclinder and piston can be modded to act like a free swinging air pump.
Only question that really remains would be how to match the mechanical system with the electircal?
We have two seemingly independent masses, one to make things move violently, the other to energise the part that make the first mass move - or to be precise to give it a tiny push in the right direction.
What does mean you wonder...
Let me try to simplify it down a bit.....
If you push someone on a swing you can have a very hard time by working against the mass of the person sitting and enjoying the ride.
Or you could give the person just a tiny push - right when the swing starts from the highest point to go back.
Could that mean it is really as simple to build as Tesla claimed?
If the top mass is limited by how far it can swing FREELY before the air pressure works against this movement then we would only have to set the length of the pendulum so the contact happens right at that point on the x-axis of the top mass.
Ok, and how would that make anything swing at all?

If the top mass is first energised there is nothing to stop or influence the movement, except the "air springs".
So it totally overshoots like those metronomes on the board.
It swings back and closes the contact on the other side of the movement way early as well.
Now the electromagnetic impulse from the second movement actually works against the moving top mass.
And this swinging of total non-control continues while whatever the thing is mounted to also starts to move, even if it is just fractional.
Over time both swings, from the machine and the object, will get to point where they influence each other in a "positive" way.
For Tesla's experiment one would say negative though as it was quite violent in the end.
Little by little both swing will get closer to being in sync and with that in resonance.
The electromagnetic push however continues....
Only that now the top mass of the machine is in sync with the thing it is mounted to.
The machine became you standing behind a big swing and trying push someone higher and higher....

It is all just a theory though and any minor earthquakes that might register around my area are pure coincidence ;)


The forums are retiring in 2021 and are now closed for new topics and comments.

1 year ago

Here is a little experiment that does not cost too much and can be quite entertaining:

Required parts:
# Shaker torch - these checp lights with a big magnet inside that you shake to charge.
# Some soft foam
# enamelled wire to wind a new coil
# the usual tinker tools and if you like a wire cutter for the foam
# A cheap audio amp

Take the torch apart and remove what is inside.
We only need the coil holder or bobbin, the magnet and the torch body.
Rewind the coil to create a 4 or 8 Ohm coil suitable for the misuse as a "speaker" on the amp.
Try to have the coil as wide as the magnet and to wind it clean if you can.
The thickness of the wire depends on the power of your amp, I just used what I had.
You need to find a balance though.
Ultra fine wire only needs a few turns so to say but won't cope with 25W from the amp.
Too thick wire means your coild might not fit back into the torch and you would need to somehow remove the outer shell...
Make the coil connection to the outside by drilling some big enough holes into the outer body or by letting them go though one of the ends of the housing, whatever fits better.

Take a really good look at the tube where the magnet is going in!
In almost all cases you will find vent holes so the magnet can travel freely.
They need to be closed up!
Same for the open end(s) once completed - hotglue will do if in doubt.
The foam is to be cut so that one piece fits in each end to SUPPORT the magnet in the center.
Support means the foam should be soft enough to allow the magnet to move out at least half way - out of the coil that is.
But not fully it you turn the torch vertically.
With that sorted make a holder for the torch so you can clamp it properly onto other things.
I was lazy and used pipe clamps and a wooden board...

If you securely attach the torch to lets say your dining table and connect your dirt cheap amp then it is time to play some tunes.
Preferable using you phone or a dedicated singnal generator.
Start with amp and phone on the lowest volume setting!
Set a frequency of about 1kHz and crank the phone volume up to about half.
Adjust the amp volume to get a clean signal.
Hint: It really helps to have some old speaker on the other channel of the amp ;)
Find the setting of maximum volume where there are no distortions, then do youself a favour and disconnect the speaker before moving on.
Our swinging flashlight won't do anything at these frequencies as the mass of the magnet is just too big.
But when start cranking down the frequency you will reach the point where either the amp craps out of things start to happen.
I have a very old phone and using a signal generator app just gives what it is set to on the headphone output.
It craps out at a bit over 26kHz on the high side and goes below 10Hz on the low side.
The amp was only 15 bucks for 25W claimed Watt per channel and does not have any filters at all LOL
Anyway, for my table it was around 40Hz when I first notice or better felt a distinct vibration.
At this point I cranked the volume on the phone up to max and the amp until the tiny 4 LED power indicator started flashing the last light.
And wow, what a difference it made....
But being about 30 years old and extendable meant it was not a perfect target.
Did not want to screw into the floor boards so I clamped my lamp onto a door frame instead and tried again to work my way dow from 1kHz.
Do not forget to crank down the volume for the higher frequencies!!
First round I had the torch vertically.
And apart from some resonating sounds throughout the wooden construction nothing much at all happened.
Turning it 90° though was not a good idea I guess.
Like this the frame is pushed sideways and transmits this swing onto other the remaining structure.
Quite easy to make all sorts of things rattle and move at various fequencies in the range of 8 to 40Hz LOL
Not possible to do any real damage but there is room for improvement ;)

I am thinking of using or making a suitable plastic tube to hold three magnets.
Like the torch but with thin disk magnets keeping the big magnet balanced in the coil.
A bit like a perload system with the disk mangets "pushing" rather hard on the big magnet between them.
The big magnet shall still be able to make tiny movements with very little force but shall require at least 50 grams of force to move fully out of the coil.
The tube will be evacutated with a vacuum pump and sealed when done.
Power source for the magnet will be a salvaged 100W speaker coild from a subwoofer.
What does not fit right away will be created in the 3D printer ;)
What are my expectations?

A vacuum system won't have any resistance andes it "free" in Tesla's terms.
At least as indendent from friction as I can go.
The magnets shall substitude the "air springs" Tesla used as with the disk mangets IMHO you get as close as you can to a spring free system in mechanical terms.
The thing I have not fully figured out yet is the friction in the tube from the moving magnet.
Guess a telfon grill liner and some lube have to do for testing.
Any next thoughts if this design shows promise?

Of course ;)
Like two ring magnets holding the moving magnet like the magnet toys that seem to levitate.
But if you ever tried to balance a magnet between two others without using and type of fixture you know the issue at hand.
Plus it creates the new problem of how to keep it working once it properly moves.
You can only go so far before a magnet system like this goes out of balance.
But if there is a will then there shall be a way....


1 year ago

I had some beer last night and got a stupid idea...
Friction free moving objects are hard to create at home.
But I had some speakers...
These fancy transmission speakers you clue onto some flat surface so the entire surface becomes a speaker membrane.
I also had a matching 25W amp of the dirt cheap kind with an input for phone....

Theory behind it:
If we glue two of these speakers together we get a nice "sound swing".
I did not want to ruin the plates that I still need for my speaker project, so I used a short bolt with a nut as a seperator.
Now both speakers could work in a linked configuration.
Sadly I failed to realise I had no proper mount now, so I had to quickly solder something to the nut to acts as a stand.

The speakers were connected to the amp with a 180° phase shift, meaning one was connected correctly, the other in reversed polarity.
Amp was set to a mono output.
At first and starting from about 1kHz it seemed not to do anything at all.
Had it standing on my tiles but only got this high pitched 1kHz noise.
Cranking it down on the phone provided some weird effects.
The thing started spinning at some frequencies for starters LOL
Going down further and it started to sound more like a tiny jack hammer, jumping off the tiles.
Then my solder connection gave up...
Fixed that and used some double sided tape on the stand to get a firm connection to the tile.
At around 35Hz and well below the stated specs of the cheap amp my entire floor started humming.
Could not really hear it but feel it throughout my body.
With nothing to loose and my micrometer barely registering and movement of the speaker ends I cranked phone volume and amp to the max and went further down in the frequencies.
Mind you the house is old and has no concrete slab, just wooden framework with some bricks around.
I went down until the effects suddenly stopped - either the amp or the phone were unable to handle things below 5Hz...
Needless to say that I had a lot of fun doing it.
I got jumpy a few times when I noticed rattling noises from my cupboards or a window frame squeeking.

So, you used massive speakers then?
Nope, as said, just a crappy 25W amp and small vibration speakers with a max rating of 25W each.
The things that happened did not happen randomly though.
Turned out the length of the stand plays a role here.
Only if the length corresponds to wavelength used to be on or ear enough a harmonic frequency the signal is amplified enough.
A bit like these ancient aegyptian "pitch forks" that were tuned in length for a specific frequency.
Would be quite interesting to see what happens if the speakers are mounted vertically and directly onto the surface.
Just struggle to find a suitable spring to put between the speakers that would make the top one weightless so to say.
Without that spring the bottom speaker would have to do too much work.
If I find the time and mood I will check what happens if I glue the ends of the speakers together with a matching weight on the mounting plates - I guess it will provide a higher overall force to be transmitted.