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As far as I know, perpetual motion orbital desktops like below use an electromagnet in the base.

enter image description here

Why aren't permanent magnets used on both ends instead?

Does anyone know where I could find a teardown of the base of a generic one?

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  • \$\begingroup\$ Pretty bad question, this. First you admit that you don't know how they work, and then you ask why they don't work in a different way? Then you ask where to find a teardown, obviously not a question for stackexchange. How are we supposed to know, you don't even give a product name or model number. Not to mention that there are no perpetual motion machines. \$\endgroup\$ – pipe May 13 '16 at 12:04
  • \$\begingroup\$ See especially this answer: electronics.stackexchange.com/a/145643/47070 \$\endgroup\$ – JRE May 13 '16 at 12:13
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They use batteries and electromagnets. They are NOT perpetual motion devices. They just run a very long (too damn long) time on a battery.

The lower end of the pendulum may have a permanent magnet. The base MUST use an electromagnet because it has to be turned on and off in time to the pendulum motion to keep it running.

The electromagnet is turned on for a short period the either pull the pendulum closer or to push it away.
This action makes up for energy lost in the friction of the bearings and with the air.
Without the energy supplied by the battery, it would stop moving.

Take the battery out and try it. No battery, slows down and stops - blessed peace and quiet for those of us who can't stand the irritating junk piles.

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  • \$\begingroup\$ Yes, I understand that they're not actually perpetual motion devices - it's just how they're sold. There're actually no sensors on it - how does it know when to switch on /off the electromagnet? \$\endgroup\$ – John M. May 13 '16 at 12:10
  • \$\begingroup\$ How do you like that? I found a duplicate of this question using google. \$\endgroup\$ – JRE May 13 '16 at 12:12
  • \$\begingroup\$ Great find! Thanks. So the approaching permanent magnet activates the transistor... \$\endgroup\$ – John M. May 13 '16 at 12:15
  • \$\begingroup\$ It is in effect an oscillator whose period is set by the pendulum. \$\endgroup\$ – JRE May 13 '16 at 12:17
  • \$\begingroup\$ Interestingly, the oldest such device has been running continuously since 1840 without replacing the batteries. en.wikipedia.org/wiki/Oxford_Electric_Bell \$\endgroup\$ – pipe May 13 '16 at 12:32
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Yes, many of them probably use electromagnets.

In order to keep a "perpetual motion" machine going, you need to keep adding energy to the system.

A battery powered electromagnet with an oscillator can convert chemical energy from the battery to kinetic energy in the machine. A stationary permanent magnet can't do that.

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