Magnetic Damping on the 5-0-5


Andrew Leigh
December 4, 2012, 12:37 AM
How does the magnetic damping on a 5-0-5 work.

You have permanent magnets mounted inside the body of the scale, either side of the groove that accomodates a piece of flat copper mounted to the beam. We know that copper is non-magnetic.

So the permanent magnets generate a magnetic field which can be felt when inserting a ferrous metal, however the copper tab which travels in the field is non magnetic, I tested it with another permanent magnet to see if it was not pehaps an alloy or merely copper coated.

How does a magnetic field act on a non-magentic object?

What am I missing?

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Andrew Leigh
December 4, 2012, 12:53 AM
Forgot about eddy currents!

December 4, 2012, 01:17 AM
IIRC that Copper bar is actually a bi-metal strip so the magnets do effect the movement but the Copper on the outside prevents the strip from attaching itself to the sides of the scale. Look under the base and you will see the magnets braced on both sides of the channel. (I Think)

Andrew Leigh
December 4, 2012, 04:54 AM
Done some reading, probably should have done so first.

Whenever you pass a conductor through a magnetic field you generate a e.m.f (electo motive force) this is a minute voltage that is generated in the conductor (the copper tab thingy). This e.m.f leads to a small magnetic field being created around the copper tab things which then reacts with the main field created by the permanent magnets. The field is too weak to force the copper tab thingy into a specific position but is is strong enough to reduce excessive occilation on the beam.

December 4, 2012, 06:48 PM
You have the metallic but non-magnetic vane thingy (copper or aluminum) basically right. When the beam's movement slows the eddy currents decrease and when the beam stops the counter EMF electrical field dies. The dampening magnet's field has NO effect on a stopped beam.

(I learned a few things I still remember from AF electronics school in '59! :D)

T Bran
December 4, 2012, 06:57 PM
If you want to see something similar drop a strong magnet down a piece of copper tubing.
It is pretty interesting to see the effect of a magnetic field versus gravity.

Andrew Leigh
December 4, 2012, 11:42 PM
Yip, pretty cool actually, always thought it was a sales gimmick thing but no. Always great to learn something new.

What I also did not realise, but which is obvious, is that as the occilations decrease so to does the e.m.f reducing the effect until there is zero effect when the beam stops. Pretty neat.

December 5, 2012, 02:10 AM
And all this time I just thought the magnets just made it work.:) I thought occilation only worked in strip bars...Maybe not, maybe thats a brass pole not copper. It's just good to know that the magnets work.

December 5, 2012, 05:04 PM
It is the movement that creates the eddy currents that dam put the movement.

It is an excellent 'trick,' and far better than the old oil dash-pots.

ANY conductor moving in a magnetic field has currents induced that oppos ethe motion.

I have a large 'super' magnet that you can use to move a piece of aluminum plate (1/2 inch thick about 4 x 6 in) across a smooth surface.

The magnet moves more than the plate with each pass, but it moves the aluminum in a series of short steps as you swing it over the aluminum.

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